I would go with a single muffler due to the fact that dual mufflers will move the powerband into a higher rpm range. These sixes do not like to turn high rpms. If you plan on racing the truck, and therefore will be running high rpms, then go with the true duals. For street use and a load hauling, go with a single muffler.

Never hold your farts in. They travel up your spine and into your brain...that is where crappy ideas come from.