Micar,
I've got 2 1/2" Flowmasters on my '72 behind Hooker Super Comp headers and a full 2 1/2" inch dual exhaust (no balance tube). They are LOUD! It seems that you don't get the typical "tinney" Flowmaster rumble if you don't (or in our case, can't) run tailpipes behind them. I am definitely NOT against a loud rumbling exhaust, but on my convertible they were just too much with the top down. If you are running stock exhaust manifolds and a cat converter, they will probably knock the noise level down somewhat. I inserted some straight "chambered" exhaust sections about mid-car and that toned it down a little bit. Still not entirely happy with it, but not bad enough to change it right now. Got a lot of other stuff I'd like to do!
I have had Dynomax-style turbo mufflers on it in the past and the exhaust note was pretty good. I would recommend them also.
Jeff