I started out running (2) flowmaster 40 series true dual exhaust - it sounded like crap.
I then added in an h-pipe and it sounded better.
I'm currently running 3" single exhaust with a Flowmaster Super 40 series delta flow muffler and it sounds the best it has. Summit's P/N on the muffler is FLO-953046.
In order to get any decent sound out of them w/o sounding like a freakin' airplane ready to take off, you either need to run dual with an x-pipe or a big single exhaust. I like the single exhaust better.