You are seeking the holy grail I'm afraid. You can get a little better mileage with an overdrive, a T5, taller tires or shallower rear gear. Now you have to subtract for any accessories you hang on the engine or for any increase in engine size. It takes one part gas for about every sixteen parts air to run any engine, 4, 6, or 8 so any size increase means more fuel. There is one thing that will increase power and economy and that is turbocharging (not supercharging) but we're talking several thousands of dollars. If you can get your 66 with it's brick like shape near the 20 mpg mark you will have done outstanding. I do know of a 64 with the later model 2.8 V6 with all the electronics that the owner is claiming low to mid 20 mpg's.

Evan