Would like to get my 66 Cuda's MPG high to drive Nor Ca to So cal

I dove my 67 slant six to LA and back for Spring Fling. Drove down on 1 in the rain and back on 5 in the heat. Got 20mpg at reasonable speeds (under 70), and a little less at 80+. Drove it to Oregon two weeks ago, no problem averaging 20mpg, but the 1qt/100 mile oil consumption kind of killed the economic equation. New motor is the next project.