2.5 inch exhaust is fine...even to 500 hp...I have 2.5 TTI on My bee...440/030 over...6 pack with stock high performance manifolds and it works well...I have 3 inch on My 67 GTX with a 498 stroker and fitting th tailpipes and over the axel pipes is a Pain.....I have headers on the GTX and get the drone....The bee has no drone...I attribute it to headers....both have crossover pipes..one thought...the first hemi's rated at 425 hp and were grossly underrated..had less than 2.5 inch exhaust systems.