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packrat · 12-23-2003, 08:16 PM

I'm just finishing up putting a 400 out of a 74 GP into my 79 GMC 4x4. I should have done this years ago. Really simple! This truck was bought new by my granddad and he used it hard, pulling horse trailers and a fifth wheel camper. It had the original sbc 400 in it when I got it but at 155K miles it was pretty tired. The final straw was when both starter bolt holes on the block had broken out, helicoiled and still wouldn't hold, chewed up flexplate and a worn out distributor that apparently is so sloppy that the reluctor hits the pickup and shorts everything out. Anywat, I had a good running GP with horrible rust for a donor, so I thought what the hell. I bought an adapter plate from Jegs for about $55. (Had to wait on a backorder. Not a popular swap, usually guys are going the other way in order to use plentiful BOP pattern cases on Chevy motors.) I was stuck with the Chevy TH400 because its a 4 wheel drive truck. I used the GP's upper metal motor mounts with the Chevy frame mounted pieces by notching the bottom mounts and sliding them back approx. 1 inch. I used the Chevy PS pump hoses on the Pontiac pump. Used the upper and lower radiator hoses off the Pontiac motor. I already had a dual battery system so the battery location wasn't a problem.I used the exhaust flanges off the GP to match the stock Pontiac log manifolds. Looks like there's plenty of room for headers if a guy wanted to. I think I'll modify the torque converter shield from the truck since it incorporates brace rods to strenghten the tranny. I thought this was a great, easy, cheap swap that some of you might want to try. I couldn't see spending a couple thou to rebuild the sbc 400 when I had a free engine sitting right here. Can't wait to see how the gas mileage and towing power compare. Wonder what I'll tear up first?!

packrat · 12-23-2003, 08:16 PM

I'm just finishing up putting a 400 out of a 74 GP into my 79 GMC 4x4. I should have done this years ago. Really simple! This truck was bought new by my granddad and he used it hard, pulling horse trailers and a fifth wheel camper. It had the original sbc 400 in it when I got it but at 155K miles it was pretty tired. The final straw was when both starter bolt holes on the block had broken out, helicoiled and still wouldn't hold, chewed up flexplate and a worn out distributor that apparently is so sloppy that the reluctor hits the pickup and shorts everything out. Anywat, I had a good running GP with horrible rust for a donor, so I thought what the hell. I bought an adapter plate from Jegs for about $55. (Had to wait on a backorder. Not a popular swap, usually guys are going the other way in order to use plentiful BOP pattern cases on Chevy motors.) I was stuck with the Chevy TH400 because its a 4 wheel drive truck. I used the GP's upper metal motor mounts with the Chevy frame mounted pieces by notching the bottom mounts and sliding them back approx. 1 inch. I used the Chevy PS pump hoses on the Pontiac pump. Used the upper and lower radiator hoses off the Pontiac motor. I already had a dual battery system so the battery location wasn't a problem.I used the exhaust flanges off the GP to match the stock Pontiac log manifolds. Looks like there's plenty of room for headers if a guy wanted to. I think I'll modify the torque converter shield from the truck since it incorporates brace rods to strenghten the tranny. I thought this was a great, easy, cheap swap that some of you might want to try. I couldn't see spending a couple thou to rebuild the sbc 400 when I had a free engine sitting right here. Can't wait to see how the gas mileage and towing power compare. Wonder what I'll tear up first?!

Brew · 12-24-2003, 07:35 AM

Now you'll fit in perfectly at the POCI events!
I'm glad to hear it's a piece of cake, as I've often thought that the torquey Pontiac motor would be great in a truck, and I'm hoping to do the same swap sometime. If you're ever in Colorado Springs, I sure would like to see it!

67 LeMans vert w/462
68 GTO bone stock
76 455 4speed T/A

mike nixon · 12-24-2003, 10:42 AM

had one in a blazer years ago. it ran well, got decent mileage and had excellent torque.

i swapped the output shaft from the truck to the car trans.

i also had headers, plenty of room for them in the truck chassis.

the chain in the t/case went on mine first. it was a good excuse to switch over to a gear driven unit.

mike

PONCHOMAN · 12-25-2003, 07:21 PM

I did a similar swap, I put a 455 with 16 heads into an 85' Chevy 3/4 Ton 4X4. The truck has headers and 3" pipes, and a TH400. It needs cab work and a timing recurve kit but it ran a 16.1 at 85 mph, at 5,000 ft elevation. I didn't think this was too bad for a big, non areodynamic, brick.
The headers are a lot of work and if you are considering a swap the only problem is clearence over the front diff. I ended up customising a pair.

sdbob · 12-26-2003, 06:18 AM

When I swapped my first Pontiac into GMtruck out of necessity it was exciting because I wanted a challenge. Besides I didn't have extra Cebie parts. I have only done 2whl drive but the results were good. Better torque,better mileage,and an engine I like. I used the stock 70 up steel engine mount. Left the truck insulators in place but loose dropped in the Poncho with long tailshaft 400 and started bolting everything up.Used longer battery cables my battery tray was opposite of starter, and cut driveshaft I think 5/8". Truck was 350/350,the gear selector wouldn't go into low right,(P,D,R, worked fine )but the swap 'had to work' over a weekend.Maybe the engine could have gone back a little or different trans linkage used.It is still in there today 10plus years. I was concerned with throttle linkage and distributor clearance.I used a stock exhaust system. My truck has a utility body on it and weighs approx. 7000lbs all the time. I noticed the big difference when pulling my tandem trailer,the torque was there. I used a stock converter with TransGO shift kit. I have another truck I want to do, the Chebie M40 is in good shape. That adapter does it work well? If so then I will buy the adaptor and leave trans in place.

PONCHOMAN · 12-26-2003, 06:06 PM

The adapter plate would work ok, but I felt that it was very flexable (prior to installation), its about 3/16" thick. The Pontiac bellhousing pattern is wider at the top than the Chevy. When I attached the Chevy Th400 to the adapter plate I felt that it needed to be more rigid. Also, with the TCI adapter plate you have to run shims between your torque converter, the shims they supply are all different thickness'.
Rather than trying to find shims that are relativly close in thickness I put the Chevy 4X4 Th400 guts into a Pontiac TH400 housing, leaving off the tailshaft cover. The Pontiac housing obviously bolted right up to Transfur case with no shorting or lenghtening of the driveshaft.
If you are interested in a TCI adapter plate let me know, half price $30 plus shipping.

RUDOLFSSON · 12-27-2003, 03:22 PM

My dad did a similar swap like you guy's are doing (15 years ago).... It was a 1981 Surburban 2500 4x4 four speed with 400 pontiac and 44" tires... Very easy swap, good looking and very powerfull mountain truck...

ChadH · 12-27-2003, 03:30 PM

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by 69GTO:
My dad did a similar swap like you guy's are doing (15 years ago).... It was a 1981 Surburban 2500 4x4 four speed with 400 pontiac and 44" tires... Very easy swap, good looking and very powerfull mountain truck...<HR></BLOCKQUOTE>

Got a PIC? Sounded nice !

You picked a fine time to leave me loose wheel ........

sdbob · 12-28-2003, 05:31 PM

Thanks for the insight on the plate. I wondered what way they made the plate. I'll do the trans swap. I really don't want to mess with shims, with my luck it'll break down far away from home. Thanks Goodluck

pontiac6269 · 12-28-2003, 05:46 PM

Would this swap be pretty simple in a 69 Chevy C10 2 wheel drive? Ive been thinking about putting a 400 with a turbo 350 to replace the original 307 , 3 speed combo in it now.

62 Grand Prix
400,Turbo400
2.73 Gears,4040lbs
13.866 @ 100.58

69 GT0 (still in the garage)

mike nixon · 12-29-2003, 05:58 PM

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by pontiac6269:
Would this swap be pretty simple in a 69 Chevy C10 2 wheel drive? Ive been thinking about putting a 400 with a turbo 350 to replace the original 307 , 3 speed combo in it now.

62 Grand Prix
400,Turbo400
2.73 Gears,4040lbs
13.866 @ 100.58

69 GT0 (still in the garage)<HR></BLOCKQUOTE>

depending on which style of trans mount the truck has it should be a drop in with the mid 70's mounts.

mike

tininjun67 · 04-23-2004, 11:44 PM

Ahh yes, nothing like a pontiac motor to tow your pontiac to the track with. I have a 400 from a 76 GP in a 65 step side that I use as a daily driver. It has a 4L80E behind it and 3.42 gears out back. At 80 mph it cruises at 2200rpm. (texas, we drive fast here all the time) There's nothing better than popping the hood on your tow rig at the track with a poncho under it. Bottom end torque really helps get me going from a stop with the 3.42's. I'm thinking of turbocharging a 350 (pontiac of course) to help with the gas mileage but keep the towing ability.

Subfan · 07-14-2004, 07:37 PM

What headers did you all use? GTO LeMans or ????

Tanksta · 07-18-2004, 07:09 AM

Hey...I've got a 94 Silverado with 6.5 TD....anyone heard of a pontiac motor in this body style? Thinkin' bout a 400. Tanksta

Thumpin455_GTO · 07-21-2004, 08:21 PM

I have a 76 Chevy shortbox, 305 was WEAK and couldnt pull the trailer and Bird no more. Swapped in a 71 400 HO, made the mounts because the parts guys back home are morons..lol. (if its not in the computer or right for that year they are clueless) It has a set of A body headers and a TH350, only problem right now is 3.70 gears in the 12 bolt. Id rather have 3.08 because im not sure a 2004R would stand up to the abuse I heap on that truck. It will accelerate up hill with the trailer behind it, where as the 305 would barely make it up after downshifting twice.

Brutus_5150 · 07-23-2004, 06:59 PM

If you're lucky enough to find a 4x4 tranny from one of the late 70's 350 OLDS diesel trucks, that works pretty slick too. I'ts easier to get an adapter plate, though.

Subfan · 07-28-2004, 05:12 AM

Any of these truck motors built for a bit of performance? What did you pick for a cam?

I am wanting to put a stroked 400 into my 'Burb.

thunderjet · 07-28-2004, 02:12 PM

i had a buddy who took auto mechanics at a local junior college in the late 80's,he bought a 4x4 tranny to rebulid for class,turns out it was pre-drilled for the b-o-p pattern.anyone one else ever come across this style of tranny? i heard they did exist

Michael Farich · 07-30-2004, 06:46 PM

Might have been one for a 4X4 with the Olds diesel engine. Late 70's or early 80's had that option.

Been reading this thread with interest cause I might be orphaning the 455/TH400 out of my daily driver Pontiac. My 78 Chevy 4X4 has a good running 350 engine, but it's tempting to put that Pontiac in there for fun. I'd like to consider stainless or ceramic coated headers for this engine - just not sure what would fit without needing modifications. Otherwise, I'm aware of the issues with driveshaft length for TH400 vs TH350, the longer support rod on pass side of transfer case and the special converter cover with the braces that attach to the motor mounts.

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