[special-K]

About any engine will run these trucks. The transmission needs to be tough enough for the weight. And,it's the rear end gearing that determines the hiway speed as well as torque. 1:1 is 1:1,it's what ratios get you there that matter. I'd suggest an engine with good torque characteristics. Finding a "made for trucks" engine would be perfect,like a 366-427 BBC. These have longer stroke than the "made for cars" BBCs. I have heard tales of car 396s going into MD trucks and not lasting long at all. Low gears mean higher revs at hiway speed and getting going takes torque. You want a longer stroke. I imagine for a SBC a 383 would do good. I'd prefer some sort of diesel in a MD.
Transmissions that come in these trucks are best. The Clark 5spd is tried and true. That's 5 gears to 1:1,no overdrive. Overdrive transmissions are usually HD truck items and a whole lot bigger. MD trucks used 2spd rears to help give good low speed power and better hiway speeds,kinda like what a Gear Vendors unit does for lighter vehicles.