here is my motor...347 ci..pro top line heads..rpm intake 750 holley..all msd..10.5-1 comprrssion..cam is .520 with 292 dr..the headers are full lenght 1-5/8 with 3 inch collector..should i get 2.5 inch or 3inch exaust system

 

3 inch. youre gonna be making lots of exhaust, plus it looks cooler

 

for a small block like a 347 I would think a 2.5" exhaust would be plenty big enough after the collectors.

 

Not too positive but you may lose power going with pipeing that big.