backpressure and hp??

i just put on sidepipes and i know that i have way less backpressure now but my q is... in order to run 3 in piping, like i have, is that affected by how much power your engine is pushing OR the size of the engine? i plan to throw an s trim on on my built 302 and hit 450-500 at the wheels, would that suffice for 3 inch exhaust?

thanks for any info.

Drew