Fuel Economy is Too Good ...

[Floyd]

The fuel mileage from "back in the day" according to the service bulletins says 20 to 22 MPG is correct. What this tells me is your car and engine is set up correctly as to timing, carb, tire pressure etc. Also it probably does not run hot.
Congratulations, a lot of these cars are reported at 12 to 13 mpg. However they usually have typical problems of over rich mixture, late timing and other things that are wrong.
If the 4.317 gallons is correct to fill the tank back to exactly where it was when you started, you are good to go!
Just an opinion