I've spent some time reading about headers/exhaust sizes but like anything else there are two sides to each topic. I will running a pretty much stock pontiac 350, with a mild cam, 4 bbl holley carb/ hipo intake, 1 3/4 long tube headers with a 3" collector. Will 3" exhaust be too much?

Will there be marginal power lost or gained with 2.5" or 3" pipes with an X flow? I don't care about mpg or noise. The car in question is a '67 firebird, stock ride height. Just reaching out to anyone who has done this before and if there will be any fitment issues as well.

Thanks again, this forum has been great

IMO a 3 inch exhaust would be WAY too much for your application. Not worth the clearance problems you will have with pipe that big. If you had a 800 hp street monster, then yeah.

My 490 HP wild thumpity thump roller cam 455 breathes quite nicely through Doug's headers, a 3.5 inch collector and 2.5 inch exhaust.

I got 3 inch with an X crossover on mine and don't have any clearance issues. But for a 350 makes no sense to go over 2 1/4"

So 2.5" still to big? I was looking at a header back complete kit from pypes.

2 1/2" is fine. Will be good if you ever decide to build a new engine.

I agree with Bigchief. A 2.5" system will sound nice and will work well if you decide to upgrade your heads and/or cam.
Unless you have high-flow high rpm engine, a 3" system would be more for show than go.

I have a healthy cam and ported heads and my 2.5" system works well. I seemed to get a bump in mid-range power with the addition of a cross-pipe.

Okay great thanks guys. Any fitment issues with an X pipe underneath?

No problems here

Nope.
Tip: make sure the x-pipe isn't positioned right under the front universal, [of driveshaft], it'll cook the grease...

Awesome, thanks for the tip