They are an iconic vehicle. I've just been left one in a Friend's will. They can be a bit heavy on steering. Try Gordon's Dodge Forum and the Powerwagon forum as well. There are some conveted vehciles there and you may get some practical advice.
I'd have thought if you are going to keep the four wheel drive, and it would be a crime not to, your choice of box maybe a bit limited. But I do know , because I had one, the Isuzu 2.8 or 3.1 disiel engine will easily fit a Range Rover box, though the diffs may not like it. Good luck , keep us all posted Steve may not be the cheapest but I've always had good dealings with him.