Unless you are going to run NOX, S/C, or a turbo, 2" OD/1,7/8" ID is fine for the headpipes, and 2, 1/4"-2,1/2" for the rest does not impede gas flow.
I prefer strait through resonators, rather than chambered mufflers.
And the stainless steel "Ultra Quiet Design" resonators from Verocious Motorsports do a GREAT job of quieting things down.
