[jonzcustomshop]

check in the projects and builds section for joey~ap's K5 66
he used a modern frame.. the key to make the swap work is the slight body lift to get the cab to clear the 81's cross members.
I think you could fashion some sort of extention between the fenders and box to get the step fenders to work on a dually.
dont worry about the shifter, the whole tranny tunnel unscrews, so you could just find another one to cut up, and keep the original, or vise/versa.
looking forward to seeing the result!