Hey Towrig, Maybe you've solved this problem already, but I was / still am in the same boat.
I have a simular tuck , and ordered the MBRP replacement muffler from Diesel Manor. As it turned out, it was not to be an easy bolt off, bolt on procedure. Refer to my posting " Damn, my truck has a Hemerroid ".
I discovered that by looking at the placements of the hangers on the MBRP and the distance going aft on the MBRP was greater than the stock muffler. That would have shifted the resonator and last back hanger further aft and just not workable as far as I could see. Perhaps some Frankenstein alterations could have been made, or even as a suggestion came forth, to check on a Quad cab design tailpipe to be placed after the MBRP. I spoke to Diesel Manor about it and eventually returned the muffler and ordered the MBRP cat back system. I repeatedly asked ir this will fit the Mega cab and was told yes. I'll see when it arrives.
How did your quest work out?