I saw that rig a couple of years ago when I was searching through expedition stuff. It has everything you could want except a diesel engine. They went with an LT1 (EDIT: it's an LS-1, I just checked the website). I don't know why in the world you would put a gas engine in something that goes places where the only fuel you could probably find is diesel.
BTW, the Gamma Goat had a Detroit 3-53.