I have heard from the veggie oil guys that they love the Benz 5-cylinder.
A quick look on
FuelEconomy.gov
shows a 1985 300D as getting in the low 20's, but average reported from actual drivers as 27 mpg. I would think that I could get that higher with a manual transmission.
Also, that's a longitudinal engine, instead of a transverse engine, so that might make it a little easier to install in a rear-wheel drive truck as well.
Ideally, it would be nice to increase both fuel economy and towing ability through conversion to diesel, but I bet that's asking a bit much!