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

I just did a 1300 mile anniversary road trip in my duster a few weeks ago.
Average mileage at about 72-74 mph was about 16.4 mpg. Empty mountain backroads at 55-60 mph I averaged up to 17.5 mpg.

The worst mileage I got was about 14.67 mpg going from about 5000 feet above sea level to over 12000 ft being stuck behind tourist traffic that would sometimes make sudden idiotic stops. Also the temps dropped to about 45 degrees at the Sundance peak on I-34.
I have a 360 V8 with an automatic (no O/D), 255/60R15 (27 in.) tires and 3.21 gears. So I'll imagine you can do quite a bit better with that Slant 6.