Q: Will the under carriage from a 1991 D 250 gas engine (318) 4 Wheel drive work on on 1991 Dodge D250

That question would be best answered at the Dodge dealership parts department to compare the items and see if part numbers are the same. The truck is older and it may be cheaper to buy the truck you want rather than convert your own truck.It would be satisfying to do but a huge amount of work.