As a plan B, I could go with an '81 and up truck, and change the bed, or the bed skins and tailgate if that's possible. It would be my version of what I call the Frankenstein S&S truck. As for turning an '81 and up into a '79, that would require changing virtually all the sheet metal including the doors. I'm pretty miffed at the guy who was selling the truck I was considering. He told me his son decided he wanted it, then called me up two days later with a price that was twice what he originally was asking. Something shady going on there.

When I first signed on to MPW I had similar plans to swap in a Cummins engine onto a late 70's Macho or just make mine look like a 81 Macho. I already owned a good running rusty 1989 W350 with the Cummins 6BT/5 speed. Plans changed and I sold it for $2,900 because of the rust issues, Seen plenty of 91-93 CTD trucks on CL here in the Midwest sell for $2,500-$3,500 due to rust.
I've seen other 70's and 80's Dodge Crew cabs with Cummins in them. Owners said it was just a simple swap as the 72-93 trucks share the same frame front end.
So it sounds like a good Idea and I was getting 20 mpg city and 22 mpg hwy with the CTD.
Now I'm just stuck with the 1 Ton user name.