well here is what im going to do w/mine.....
straight off the manifolds in to two 3" straight pipes to the back of the cab then into 5-6" stacks up each side of the of the front of the bed i am not going to merge them into one pipe i am going to give a new meaning to "straight pipe exaust"
I bought mine truck when it came off the engine into a single muffler and then to duel pipes out that back. I went to my local muffler shop had them cut right before the y-pipe of the engine and right after the y-pipe coming out of the muffler and had them run duel pipes. Very, very loud if that is what your looking for.
I have an 06' 1500 i just put flomaster series 50hd. Dual exhaust out the sides on an angle behind the rear tires with stainless steel tips and i love the way it sounds. would definitly recommend flowmaster.