This guy put a Cummins 4BT 3.9L diesel in one:
1967 El Camino 4 BT swap
A guy I know just put a 4BT into his Dodge Dakota, and he was getting mpgs in the high 20's and I don't think he's very eco-minded.
I've thought for a while that an El Camino would be a great vehicle for an aero-cap as well!