Just make sure you have headers, long tubes as you youngin's call them (real headers as us old farts call them). This will give you the best performance gain and also have a big influence on the sound.
I see a lot of guys looking for the ultimate exhaust sytem with that mean "old school" muscle car sound. Well, our old muscle cars always had headers on them, which are a big contributor to the classic sound. To give an idea how much this means, my wife's '66 had no-name turbo mufflers on it when we bought it and the system looked relatively new, and it sounded like crap! I figured I would replace it after I rebuilt her engine, but after I got the engine back in with headers and started the car it sounded like a race car. No reason to replace the cheap mufflers, it sounds great.......then again the engine is stroked out to a 331 now.
Steve