Here's my .02 about weighing whether going 2.5" the whole length now is "worth it" in this situation.
- Peak RPM. Higher the rpm, the more exhaust. More exhaust, the greater the need for larger diameters for best top end flow.
- VE. The higher the VE, the smaller diameters needed through the same rpm range. Street hot rod engines its prob best to assume lower VEs.
- Mufflers are the main restriction. 5 or 10 feet of 2.25" pipe is generally nothing compared to the losses in the muffler.
- Use a pair of good flowing mufflers! In general a 2.5 in-out will outflow a 2.25 in - out of similar design.
- Assume when you install headers new pipes all the way to the mufflers will be needed. The angles and positions are generally too different.
IIRC '68 383 came with an H-pipe. If you do an H, put it in the right spot. With manifolds, copy the factory position.
Exceptions to my comparison of plain pipe to muffler restrictions:
* Sharp bends, cheated bends, and closely placed bends, especially when the tubing diamerter is reduced by the bending.*
In concept I'm fine with pipe diameter reductions after the muffler. The exhaust gas has cooled considerably by that point, and it would be good to keep velocity up all the way to the exit. However, in reality it depends how its done. That's why these tailpipes are coming off in the next week or so. I'm going to test a mandrel bent 2.5" tail pipe and see how it fits. I may end up splicing in some 2.25" straight pipe. Since your car is a '68, you should have less trouble with the tailpipe fitting than a '67. If you have a local shop that can make or weld up the bends, that's the cat's meow.
This gives some idea of reduction in size and turn combination that is just not going to allow for good flow. Even at mid and low rpms its probably hurts velocity.
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Driver Side is worse.
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