I must agree with the others: not the best idea. With the power available in a tweaked 4BT and the relative size/weight of your rig, plus some huge fabrication headaches for a 6BT, it's a no-brainer: 4BT!
If you haven't driven a 4BT powered rig it's hard to imagine the available torque, right off of idle. I'm very impressed with the excellent performance of a bone stock 4BT in my 6,500 lbs Dodge. I have about the same power as a Dodge 318 2bbl, and the torque of a 360 2bbl, but the torque is right there, all the time! After twisting screws and swapping a few parts it should be really great.
I don't think you'll see very good mileage with a 6BT, maybe very low 20's if that. You're dealing with weight, plus 4WD gear/drivetrain and tire resistance, and probably a fairly high number final drive gearing.
Buy a 6BT truck, strip the parts you want, sell the engine and leftovers to pay for a 4BT engine!