Just to keep things realistic, the fuel mileage of something that size with a mid-70's 455 v8 would probably set a world record if it achieved 15mpg on a level road. Monitoring the weight of the fuel tank would probably be good enough for calculating average mileage over short distances.
Older "fuel economy" gauges were often simply a vacuum gauge monitoring manifold vacuum, a heavy load or accelerating rapidly drops manifold vacuum and reduces efficiency.
You would probably be better off monitoring the ignition timing, manifold vacuum, and exhaust gases (oxygen sensor) to detect when the carburetor or ignition were getting out of tune.
You might also want to do some research to see if the carburetor needs any modifications in order to run the alcohol/gasoline mixture that is commonly sold these days (depending on which country you are located in).