To help out on the size and fit issue, i have an mbrp kit and it works fine. 4 inch downpipe, then expands to 5 inch all the way out.

 

before i had stacks i got an expandit 5in exhaust from luke (diesel dawgs) and it bolted right up... the only hard part was getting the old exhaust off. he got me a good price too.. and it was at my doorstep the same week. i only order aftermarket stuff from site sponsers.