on my charger, i'm going 3 1/2 into a 3" xpipe, then couple to the existing 3" over the axle pipe, 3" rearward from there into a race bullet of some kind, then a fabricated [by me] turndown 6" or so, wide.
if i don't like the sound, i will install something where the factory mufflers were [possibly another race bullet ?].
after it's all done, i'll send the total system out for coating.
and believe it or not, i have run a strait 4" pipe [adapted from the 3" over the axle pipe] to the bumper along side of the factory gas tank. it fit fine with no clearance issues.