Well, up front I'm a Cummins fan. Bought a new 1995 Dodge Cummins (still drive with 250k miles) and had 3 at one time. Rebuilt a few, and turned up many (with full gauge packages only, aluminum pistons do melt). All that said, I really appreciate the simplicity of the mechanical injection pump 4bt and 6bt engines. My first choice would be a 4bt with the inline injection pump. They're not hard to find since they were the engine of choice for the utility trucks used by bakery's, delivery company's and who knows how many others.
Electronics, don't need no stinking electronics with a mechanical fuel pump Cummins. Now they do require electrons for the starter, and fuel shut-off solenoid....but it's easy to replace the electric solenoid with a cable, ala tractor fuel shut-off. Pull to kill the engine, push in to start. That's what's on my 95 both for simplicity and it's a pretty good theft deterrent.
MGP, it's not uncommon for 4bt conversions to get near 30mpg. 6bt also do very well.
All the above said, the biggest advantage to the Cummins is the stone like simplicity, less to go wrong and they'll run till just before the sun burns out.
Buy a clapped out 1991 1/2 (Dodge added an inter-cooler for the turbo at mid year in 91) to 1997 Dodge Cummins, harvest the running gear and get to work.
Oh, and the easier path would be to strip the body off the Dodge, then graft the Chevrolet body to the late model frame/running gear. Benefits include modern brakes and suspension. Easier and safer.
Good Luck,
RonR
