Big fan of the Dynomax Bullets here. Have a pair of 4" bullets I use on the car at the track. Way quieter than open headers, but runs the same with or without them. Slows down w/ every other exhaust I've tried.

CHIP
'70 hemicuda, 575" Hemi, 727, Dana 60
'69 road runner, 440-6, 18 spline 4 speed, Dana 60
'71 Demon, 340, low gear 904, 8.75
'73 Chrysler New Yorker, 440, 727, 8.75
'90 Chevy 454SS Silverado, 476" BBC, TH400, 14 bolt
'06 GMC 2500HD LBZ Duramax