I've been thinking about doing a 4BT swap in my J10. The 6BT is just a little too long to do it without major surgery. I figure I could build up a 4BT with a good intercooled turbo to make enough torque to move the truck around with my CJ on a trailer behind it. Sure it wouldn't win in a race with a modern diesel truck but I bet it could hold its own heading out I-70 to Moab.
2012 JK - 3.6/NSG370/NP241/D30/D44 - Grimm Jeeper
1986 J10 - 360/700R4/NP208/D44HD/D60FF - Truck Norris
1984 CJ7 - 5.3/4L60E/Atlas 4spd/D44/9" - Work In Progress
My gear ratio calculator