Well, I am new here and know nothing of fords, my father in-law wants to put a new exhaust on his 99 crew 4x4 s-box man-tran. He wants cheapest, and better sound. His has a Cat. on it. I found a nice turbo back kit from summit but it cuts out the cat. Here in CA diesels are smog exempt but I know Fed law prohibits removal of cats any way. If we go with a 4" kit and graft in the OEM Cat how will it affect performance, and sound?? He doesn't want to spend more than 300 on a system.

PS wich is a better size 4" or 5"???
PS wich is a better size 4" or 5"???