I really can't see the 6bt getting that much better fuel mileage than a 4.0. Plus, the weight, size, and limited powerband of the diesel will make this a very unwieldy project, to say the least. The scrambler frame, aftermarket or not, really isn't up to snuff for that much weight. Engine compartment size and radiator size will be two other limitations. If you are really set on doing this, I would suggest a Wagoneer frame. Or an Isuzu turbo diesel. Or both.