3 inch exhaust with manifolds??

Nothing wrong with that at all. The stock high performance manifolds have 2.5" outlets, so stepping up to 3" shouldn't be difficult in that body. I would do it. As mentioned, "about" all you would gain is in sound......but that's a BIG difference. I've done it with manifolds and a mild Chevy 350 and it really woke the sound up. I plan on running a 3" system on my Ford truck when I put the headers on it.