As said several times, 2.25 will work with what you have, 2.5 will work up to around 475hp. I have a 427 in my chevelle that puts out around 535hp, and I chose Borla ProXS 3 inch mufflers, but ran 3 inch to the muffler, and 2.5 tailpipes. It's perfect. A big advantage in snaking tails in and...