i have a sbc 350 with:
Bored 30 over with new pistons
edelbrock performer cam
edelbrock performer intake
edelbrock performer carb (600cfm)
edelbrock air cleaner
turbo 350 transmission with b&m shift kit and B&M something converter
pacesetter headers
the exhaust will be either 2.5 or 3 true duals to dynomax bullets dumped before the rear axles. volume is not a concern. whats the best performance and sound?
thanks lucas

 

3 inch will flow more and will shift the torque peak slightly up rpms, you wont particularly notice much. the 3 inch will probably be marginally louder and slightly deeper ( my pals gto v8 has 2.5 and i have a 3 on the maro, mine is a little deeper)

 

In my opinion...BIGGER is BETTER.

 

That's what she said.
But with a fairly modest engine as described, a 2.5" exhaust is more than sufficient. 3" would be overkill, and going too big with the pipes can actually hurt performance in the lower rpm range (most of your average daily driving). Bigger isn't always better. But will you feel the difference with either size on a street car? Probably not.
A good simple rule of thumb is an engine with 400 h.p. and above will benefit from a 3" exhaust. But, if you ever plan on stepping up the h.p. to be in the upper 300's/400's, then go with the 3" and build the engine to suit.