The least aerodynamic part of a car is the stuff under it.

A belly pan woud be a fairly cheap and effective addition. Just remember to allow for air to flow out of the engine compartment, the radiator will thank you.

Back when I had my Diplomat 318 powered I could turn mid 20's on the highway. That was with 2.24 gears and an aerodynamic package at least as brick like as yours. City mileage was horrible though, even with the low first gear trans adn a lock up converter.

I later put in a 360, an A833 and a set of 2.94 gears and would turn low 20's with that combo. City mileage wasn't too bad then.