2.5" exhaust ?s

[lowbrow454]

This video explains all you need to know. https://www.youtube.com/watch?v=bXQ-cXeri1Y&t=604s

Really good video and explanation.

To add to that this was taken from the Flowmaster website:
What size exhaust tubing should I use for my street application?

Choosing the proper diameter of the tubing for your exhaust system is as important as selecting the right muffler. Your tubing diameter choice will affect the sound level and performance characteristics of the muffler, but keep in mind that bigger is not always better. Using tubing that is too large can actually hinder exhaust scavenging by slowing the velocity of exhaust pulses in the tubing resulting in a loss of torque. And torque is what moves the car. As a general rule, most stock to mildly modified V8 powered street applications will use 2.25” – 2.5” diameter tubing. There are many variables that need to be taken into consideration when choosing the correct tubing size for your application, such as: engine technology (flat head engines vs 60’s engines vs present day engines) cubic inch, catalytic convertors, head flow, vehicle weight, how the vehicle is driven and numerous others. Light to moderately modified street applications will generally use 2.5” and rarely some can use dual 3.00” tubing, again depending on the above named factors, sound desired etc. *We generally don’t encourage the use of dual 3” exhaust systems for just street use. Here are some guidelines for exhaust tubing size vs power that can be used for street applications: Street V8 applications – dual exhaust 2” tubing200 hp – 250 hp 2.25” tubing 250 hp – 375 hp 2.5” tubing 350 hp – 500 hp *3” tubing 500 hp – 600 hp

I've had both 2.25" and 2.5" exhaust on both stock and mild 350's including one with vortec heads like you're getting to swap in and honestly I didn't really notice much performance difference either way. The 2.5" sounded "healthier" but as far as making more power, if it did it wasn't noticeable.