Big block guys will correct me if I'm wrong, but conventional thinking would be 3" and think about 1 7/8" headers for max power. At your displacement/power target, I think the 2.5" would hold you back and with 1 3/4 headers you would be trading high RPM power for low end torque.
What are your goals though? Packaging, cost and weight would be the down sides of bigger tubes. Maybe noise too, depending on your goals.
F.A.S.T. Class is restricted to 2 1/2in exhaust. Several of those cars are in the 9's at well over 130mph. I doubt you'd feel any difference between 2.5 and 3 inch.
I know on my 526, there was no noticeable gain going to 3 inch.
However, going from a 850 and an rpm intake, to a M1 and a "1000" cfm carb, picked up A LOT.
Every combination is different. 3 inch WILL make it louder.
