If you do a Google search for "optimal exhaust size", you can lose your marbles trying to consider all the variables associated with the answer. For a shorter opinion, my perception is a 2" exhaust is probably adequate for a stock 350 system. If the engine has a cam and headers and is mostly used on the street a 2 1/2" system is probably ideal. 3" is more likely needed if you have a tunnel ram, spray, blower, or a big block. The trade off is loss of low end torque, which might be substantial on a stock engine.
But if the cool factor is more important than practicality, exhaust systems fall into the same category as jugs and motor mounts, too big is just right!
