My 71 Sebring with fresh all stock 318/904 and 2.73 gears with 235/60-15 tires got 22 mpg on the interstate too. We drove it from Seattle to San Fran and that was the average. I had a 93 Dakota regular cab 2wd we bought new with the magnum 318 auto. It also would get almost 22 mpg with its 3.75 gears and overdrive if you kept it at 65.
Personally the car I'd like to see tried is a slant six, 4 speed manual overdrive, and simple fuel injection setup. Basically a feather duster with fuel injection. Maybe add better exhaust, lightweight wheels, and do it on a lighter 68 Valiant. No reason a car like that wouldn't get 30 mpg highway.