[66sprint]

I am planning to swap the worn out 350 ST300 2.56 ratio (assumed) rear for a mild 455 200R4 and 3.55 rear.

I really do not want to mess with headers or wait on the long branch manifolds (and the cost of those).

So my question is if I re-use the 350 exhaust manifolds, will I really be giving up that much?

And being a convertible is there a good option for fitting muffler[s]?

I am OK with using either a decent size single exhaust with a crossflow muffler behind the rear axle or possibly a dual exhaust with a couple of turbo mufflers ahead of the axle. However it looks like they will be difficult to fit in there with the cross brace.

I intend this to be more of a cruiser than a performer.

Any suggestions?

[Lee]

I did a chassis dyno test, comparing the stock manifolds to a set of tri-Y headers on my mild 455, in a '73 Firebird. The performance difference was surprising to me. I had corrected the AFR with the stock manifolds, and re-corrected when the headers were installed - so not a case of skewed results from lack of optimum tuning.

There is plenty of room for mufflers, the problem is routing the pipes to them! There are some different braces (the part that bolts to the floor behind the trans) out there, that give more clearance.

I also recently installed a set of Doug's D569 headers on a '68 Firebird convertible. I made a few modifications to the headers, but the install was surprisingly quick & easy. I actually took some video, to show what mods I did and how easy it was: https://youtu.be/EXBkWpQJJJU

[66sprint]

Lee thanks for the info and video.

I am really surprised by the difference. I would have guessed 10-15 HP and Lb/Ft at most.

[JLMounce]

I just had a set of dougs headers installed and the fit is very good. They tuck up nicely to the floor and do not hang below the subframe.

The only issue you really have with those is your standard sized filter does not fit on the oil filter adapter. You have three options for that.

1. Run a smaller filter. For a mild cruiser 455, that's probably the route I'd go
2. Run the standard sized filter with a nylon reusable gasket for the filter adapter, and remove the whole thing each oil change
3. Install a remote filter