I suspected as much, probably an LD (not LDT, T=Turbo) non turbo truck from an early A2. The non turbo engines smoked, were kinda low on power (but probably shouldn't be that slow) and they added turbos later with the A2s to clean up the smoke. I was with a convoy this weekend and the non turbo truck smoked a
lot more than the rest.
If it's a '62, it should be an M35A1 with the LDS 427, which had a turbo, but smoked a lot and was supposed to make good power...but was unreliable in the headgasket department. A1s had horizontal exhaust, not a vertical stack like A2s, and I don't see a stack on yours. I wonder if the military ever updated it to A2, or if it got out of the military before being updated. I'd be interested in seeing more pics of it, interior, dash etc.
Does it idle smooth or have a "burble" to it? Can you hear valve tick? Here is mine after valve adjustment. Has turbo, but I think yours should sound pretty smooth too.
https://www.youtube.com/watch?v=5AmtSLRfmFQ
One way or another, smoke means more fuel than air, and low power with smoke, probably more of the same thing. These other guys are more experienced, (esp with non turbo engines) and could probably comment as to turning up the fuel for more power, but that's just going to be more smoke. I imagine a turbo could probably be added to that engine, but the injection pump might have to be changed or retuned to match.
I don't know what kind of funding you have available, but an upgrade to a Cummins 5.9 might not be a bad idea. More $, but newer, lighter, more powerful, modern, available parts people are familiar with...but no cheap or easy task to acccomplish. At that point, replacing the whole truck might be more cost effective.
Barring that, this forum can help you get that truck running like a top.
http://www.hanksdeuce.com/cummins_engine_install.htm
