In 1986 the original owner of the truck had a Cummins shop install a 6BTA. It looks like they used the original 5 speed. Someone later changed the ring and pinion to take advantage of the Cummins' rpm range. It will easily run 70 mph+.

The issue is the trans. The original 5 speed doesn't seem up to the diesel. This was confirmed by the original owner. How much trouble would it be to swap in a Fuller FSO6406?