318 MAX fuel economy builds?

In the mid 70's I drove a 1970 bare bones Duster 318 4 speed 2.94 gears with the tallest rear tires I could find. The engine had a factory spread bore 4 bbl 340 manifold with a thermoquad,secondary's disconnected,headers and full dual exhaust. I would drive it from Pittsburgh Pa to Denver Colorado ruffly 1600 miles.I woud do it 4 times a year.I would average between 24-26 mpg running the then speed limit of 55 to 60 mph.Weight is a big factor.