I've got the big 3" Super Turbos in my '66 GTO (and Doug's round-ports) but the car was still way too loud for me. I then installed those little "resonator"-looking things:
http://www.dynomax.com/mufflers/race...und-24405.html and those made just enough difference to make it tolerable.
Also, just looked at the Dynomax website and it looks like they no longer have the long-case 3" offset/offset that's the correct configuration for '64-'72 A-bodies... might want to call them to verify.