I have a 6.2 non turbo/spicer 3053a tranny/200 t case/stock axles/xl tires at around 4800 feet. My experience is I think the 6.2 is ok, I'd prefer the 6.5 and also believe a turbo should be manditory on the 6.2 if you go that route. On the highway the 6.2 at around 3 grand is running 63 mph according to my buddy's GPS, and that's with the overdrive in the tranny. Certainly going to the CUCV axles would help, and is my plan for the future as well as divorced 205.
I'd stick to the 6.2/6.5 with the stock drive train, if you want to go diesel. The Detroit engine sounds like a lot of work for little if any gain- gonz
Come and take it
Go work at joann fabrics if you can't shoot a gun