I cant help you find out anything about your truck but I can tell you that my father bought one brand new in 1978 at riverhead dodge in riverhead New york and was told by the same dealer later in the 1980s that dodge never made a truck like that, when my dad showed him the decals on the front fender that said Diesel Power, the dealer said that he must have put them on him self. I have heard that the mitsu engine in it was used heavily in the marine industrie but I dont know for sure.
do a model search for 6DR5. Its seems to be the same engine, but for use in a front end loader. In the truck its a 6DR5 "A" but this is a 65R5 "C" - wateer that means.
I have a a 1978 Dodge with a 4.0L Mitsubishi Diesel also
I work at a parts store and a cutsomer of mine is having a hard time finding a water pump... and it IS for a 1978 Dodge Truck 4 speed with a 4.0L (243) Mitsubishi Diesel w/o turbo and it is a 1/2 Ton. If anyone has any information on where I may find one I would greatly appreciate the help! And yes they do exist.