[POWERSTROKE]

My Father-in-law had two 6.2L's. One in a GMC loaded conversion van, about an '82 I think. Then he traded a 4wd Ford Ranger he used on his rural Post Office mail delivery route for a mid-'80's GMC K-1500. The Ranger had the 3.0L gas V-6, and the 6.2L diesel got substantially better mpg, that's one thing they did well. They had problems running over 100,000 miles, various problems. I'd buy two, one to run and a spare. My neighbor had a 6.5L GM diesel in his farm truck, think it was 3/4 ton 4wd, He had a spare rebuilt engine sitting on a skid in his shop in a big plastic bag ready to install. I think it was his original engine. SON drove the truck a time or two, He said my 7.3L PSD ran circles around it, but like the 6.2L, it got better mpg. The 6.5L did make more HP than the 6.2L, think the 6.5L was a bit more reliable too.
I still think a 4BT Cummins would be a good swap into a '60's GMC, maybe a 6BT even. Just a rear axle swap or gear swap gets you great highway gears for cruising with great mpg. SON got 20-21 mpg with his 2014 RAM 250 CTD 4wd mega-cab with auto trans. It ran about 1400 rpm at 70 mph. If he slowed down a bit it got even better mpg. The early 6 & 12 valve Cummins are kinda expensive to buy just an engine. Lots of people want them.