Here's a low-compression 400 that I put together on the cheap.
I started with a 1969 400 engine that came standard in the Grand Prix that year, this one had around 90K miles on it. This one suffered the same fate as many others with the factory nylon upper timing chain sprocket. The teeth came off the sprocket and the next thing that happened was the valves started hitting the pistons, bending nearly all the pushrods. Beside maybe a couple of slightly bent valves, the rest of the engine was in great shape.
1969 400 block, still std. bore as the cylinders had almost no wear ridge, this engine must have had some decent care during its life. I reused the original pistons and put some new std. iron rings on them. Pistons sit about .020" down in the hole like most factory 400 engines of that era. The original rods were reused with new std. bearings, they checked out good. Same thing with the original 400 crank, new std. bearings installed and factory balance job retained.
1976 factory baffled oil pan with HO Racing trap door mod to retain oil pressure on hard right turns. Melling M54DS oil pump, pickup retained with a 10-32 screw through the pump housing and tube and loctited in place, HO Racing hardened oil pump driveshaft. Stock filter adapter with bypass blocked off using a 3/8 pipe plug.
1976 6X-4 350 92-94cc heads, no milling or porting/port matching. Rocker studs were replaced with 7/16" big-block Chevy studs and stock Chevy self-locking crimp nuts were used to set the valves. Stock valve springs were reused as well as the retainers and keepers. 1976 valve covers with the drippers were used to help keep the rockers lubed.
Compression works out to
8.2:1, I put the original #62 high-compression heads off to the side.
Edelbrock Performer cam (204/214 @ .050, .420"/.442", same as Summit 2800 cam) was used with stock 1.5 stamped rockers and stock pushrods. Cloyes Tru-Roller timing chain, cam installed straight-up.
1972 factory iron Q-jet intake and a 1972 455 750 Q-jet were used, the exhaust crossover was not blocked off. Topped off with a stock 1970 GTO twin-snorkel aircleaner.
Stock log exhaust manifolds with the outlet ID opened up from 1.900" to 2.100" to match up to the ID of the 2-1/4" headpipes. Full 2-1/2" mandrel-bent exhaust with H-crossover, Walker Dynomax 17749 20" long Super Turbo mufflers.
TH400 trans with Trans-Go shift kit and stock converter, stock 3.23 open rear and 235/75-15 (28" tall) whitewall radials. Engine resides in a 1969 Grand Prix, weighing 4100# w/o driver.
Best ET 15.11 @ 92.7 MPH, launching fairly easy to keep traction. Not bad for low-compression 400 running nearly all stock pieces, easily a high-14s car with any decent traction tire. I was getting 13 MPG with this car running at 80 MPH on the freeway with the A/C blowing ice.
An honest 270 HP 400 built strictly for commuting, runs hard on 87 octane. 
