i had stock manifolds/cats, with a LM, sounded awsome, i loved it, it was loud to the point that i could hear it standing at the end of the driveway( which is about two F-body lenths long), crusing down the road it made noise, enough to turn heads and if you were easy on it the cops really didnt bother you, now if you get on it, people can hear you two blocks away and when the cop pulls you over, hes gonna say its "ungodly loud", well thats what he said to me
now if you gonna go with headers, might i suggest my current set-up, True Duals, very deep sounding and loud at all RPMs, even when im trying to be quiet its very hard, sounds beautiful, the way a muscle car should sound like
if you want to stick with the Y-pipe set-up id say to go GMMG, sounds great with or with out headers