Grand rock makes the stack kits
but also from experiance, buy the down pipe from mbrp, then make your own stacks, the kits put the stacks at the bed rail which is to far out.
I put a kit in a 03, and did more modifying to the kit than what I did fabbing a kit from scratch.
as far as HP gain from a 4" vrs a stock down pipe on the dyno you will see none. hell a bunch of us are putting close to 350hp down in obs's with 3" dps
as far as driveability there is the difference, the 4" allows to spool faster, also keeps egts down .
as for the muffler , I recomend especially on the longer wheel bases or the drone is incredable.
My 03 is stock as far as power and I put a 4" dp and duals on it.
now its ready for any other mods.