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that 2.8 would have a hard time moving your heavy truck, and all its complicated electronics would be a BIG issue, true it does have 295 ft*lbs of torque, but its not at a very usable part of the curve for your heavy truck, and hp is on the low side, not to mention they dont really get that fantastic of mileage even in the lighter jeeps

your best bet is to find an old delivery van with a 3.9 4BT cummins, it is exactly the same as a 5.9 12V cummins, with the front 2 cylinders off, they have decent power, easily upgradeable with many of the same components as the 5.9 12V, and is completely mechanical, you have hardly any wiring to do, also it comes attached to the TH400, so hooking it to another GM trans would not be too difficult, hardest part would be motor mounts

allison 6speed would take a bit of wiring for all its computer controls, although it is a good thing it has a stand alone tcm, also not sure how it would mate up to whatever engine you choose, although adapters can always be made