Andy...I use the small tube headers with mufflers. I have an FFR with shortie headers and the "J" pipe...so I guess I would be closer to accurate with what I'm using. If your kit uses headers, just figure small...or large tube (you can measure if ya want!) with mufflers. I think every kit produced for the street has mufflers of some sort. Oh, and by the way...my 397 HP would jump to 437...and my 393 ft lbs of torque jump to 408 using the large tube, open header theory!!! Boy...wish it were so!!!
dino