you can buy an x-pipe made for LT headers, or get the one you have cut to match.. any v6 headers will go with almost any x-pipe, and then just make sure the pipes are the same size in the x-pipe and catback (2.25", 2.5") and it will all match up
With my old V6 i ran str8 pipes....no mid pipe and it sounded pretty sweet for a 6, i also had magnaflow catback and i had many comments on it, everyone thought it was an 8, except those with trained ears of course to know the difference.
the mid pipe really isn't needed anyways for the 6, it is a pointless mod, i would invest that couple hundred bucks on something else and just get the y pipe cut.(reason being the 6 has an even firing order and doesn't need the exhaust pulses to be evened out by the midpipe.)