4bt-3.9L 4cyl turbo diesel. came in box trucks (UPS, FedEx, bread trucks, etc) bolted to either a gm auto or the ford 5 speed manual. th400, th475, and 4L80E were the gm auto tranny's used. motor was also used in oilfield applications and some construction equipment. very reliable motor, basicly the 12 valve cummins 6bt with 2 cylinder chopped off.
12 valve 6bt-5.9L 6 cylinder used in (i think) 1990-1997 dodge 2500-3500 diesel pickups. extremely easy to build up this motor just like the 4bt because of the mechanical injection. simple, reliable, and powerfull. also in some oilfield/construction equipment.
24 valve 6bt-5.9L 6 cylinder used in 98-07 dodge pickups. electronic injection so requires a computer. little more prone to problems then its 12 valve predecessor but still an excellent motor.