[66Submarine]

I think upper-teens is pretty easily achieved with a half ton (IME), at least if you aren't driving real fast. (Haven't been able to test how much of a hit you take from higher speeds, so I can't say for sure there.)

IMO, a 250 six or 283/327 (perhaps a 302?) with a manual OD trans would be at the top of my list if I was putting something together for economy. A good ultra-common 350 also works well.

Personally, I don't find these trucks to be really thirsty.