3" exhaust? 2.5" exhaust...??

Up to 450 hp a 2.5 system will be more than fine. Smaller primary tube headers will build more torque, but lay down on the top end. larger primary tubes on the same motor may be dead on the bottom but allow for more rpm. it is all based on the combo of the motor etc rpm you plan to run and parts etc HP goal. Its a recipe if you will some work some dont.