I have 2 single in single out 50 series flowmasters. The muffler shop just cut out the stock muffler and welded in 2 SISO mufflers up and down beside each other. The entire job ran me like $130. The sound is crazy outside the vehicle, and is bareable inside even at highway speeds. It is loudest inside the cab at 2700-3400 rpm. Loudest outside at 2200-3100 rpm.
And I even got a little more low end torque.
well here are a couple of pics. Sorry it's dark and my rig is dirty.
http://i756.photobucket.com/albums/x...rexaust014.jpg
http://i756.photobucket.com/albums/x...rexaust016.jpg