I would not think a Dart Sport / Duster / Demon would be too bad, with the swept line portion of the rear roof. The Swinger / Valiant / Scamp would be a little worse IMO since they are a little more boxy, and less smooth coming off the rear glass and trunk area.
Granted, the nose are is pretty significant, but in a max MPG effort, there would be some aero modifications made, full or partial grill block, under belly pan, etc. However, if a absolute max MPG build was the plan in the first place, then the person would not have started with an old school mopar...