I would suggest Hooker Aero Chamber mufflers. Great balance of flow/power and not too loud, but a great sound.

Dynomax Super Turbos are real popular, but VERY restrictive.

Dynomax Ultra Flows are great IMO, but a little loud for most folks.

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