The easiest conversion would be the Chevy long bed K20 into a CUCV. The Chevy Blazer might be tougher as I haven't seen the correct body style in plastic. After that, it gets tough. Possible a Jeep Gladiator into a M718 (Not sure of the exact number). While the Dodge Powerwagons were close cousins of the M37, the bodies were largely different. I was looking for a M37 for years in 1/35(or any other scale) and Roden finaly made one a couple of years ago. I have a 1:1 M37 and have a pretty good idea of which parts are interchangable with the Powerwagon.
Support vehicles used by the military were often close to their civi counterparts, so it would be on a case by case basis.