I've got plain ol' Maremont brand Cherry Bombs. They make some that are just about the perfect size and offset to go in place of the stock mufflers on these things. Not to mention they cost $22 each from the local AutoZone. There's a write up on my CarDomain site:
http://www.cardomain.com/ride/364830/2
I'm not a big fan of Slowmasters. They flow better than OEM mufflers true but there are many different mufflers out there that flow better than Slowmasters. That and EVERY redneck around here with a V8 powered pickup has a set of dual tailpipes and at least one Slowmaster 40 series. I guess I'm tired of the sound...hell, I even had one on my Blazer about 5 years ago.
If you want to spend $60+ each on mufflers consider Magnaflows. Straight through like a Glasspack/Cherry Bomb but also stainless steel. I've put them on some of my vehicles and love them. The only reason I didn't go with them on Big Ugly is I am sort of considering a turbo and a 3" or so single exhaust so I figured I'd just use Cherry Bombs until then. Magnaflows sound good, cost the same (or less in some cases), and have that great stainless steel finish that holds up really well. Mine each went through two Michigan winters exposed to slush, snow, ice, and lots of road salt and both looked great when I last saw them (sold my previous pickup and the wife's SUV was in an accident).