The 6.2 or 6.5 is your easiest and most practical swap.
Go for the later engine with a turbo and power is acceptable, but not spectacular.
Any more engine both power and weight wise and you should be looking for a 3/4 or 1 ton frame and axles.
Many people have done larger diesel swaps on small trucks, but I don't like the idea, I guess if you never load the truck it's one thing, but for a useful truck that's another...
A Cummins 6 cylinder from a Dodge is probably the best swap for power, reliability, parts availability, and bolt on / plug in horsepower potential. A tad heavy for a Dana 44, as Dodge had their own bunch of problems with the Dana 60 under the Cummins.
I like Detroit's, check out my two converted trucks on the link below.
Grigg
