The conversion from 2WD to 4wd has been done on D series Dodge trucks but most agree it is not worth the effort and gets more expensive than it's worth. Find a beat 4x4 and swap your body and engine in or just sell the 2WD and buy a 4WD. If you decide to swap the body over, make it easier on yourself and get a truck from 1981 to 1993. The '70s models are workable, but most are full time 4WD, have weaker front wheel bearings and the half tons have the small bolt pattern wheels and the earlier models are divorced transfer case and front disc brakes are rare. The easiest swap would be by using a 1988 to 1991 W150, W250 or W350 as they are mostly V8 models with TBI like yours. The '92 & '93 models have 318 Magnum engines (360 Magnum is '93 only) and pull a premium used.