Ground post and terminals are one of the most overlooked wiring harness elements and they can be related to most common malfunctions.
Any DC accessory on your car needs a good positive polarity source as well as a ground to complete the circuit.
As grounds are not directly connected to your battery negative post (because of weight saving measures), but instead connect through your car body, their good operation depends a lot on the cleanliness and absence of rust on post to car body and terminals.
Your classic is not anymore a youngster, but an engineering marvel with 50 years or more. Time passes, and the accumulation of contaminants (oil, paint, rust, you name them) on grounding surfaces affect dramatically their performance.
If you observe dimmed lights, erratically fuel gauge behavior among many many other gremlins, pay attention to your ground post/terminals. They might be the culprit.
Disconnect them, polish with extreme care contact surfaces. Finally in order to mitigate rust formation over the surfaces you just polished, it is a good practice to apply a small amount of conductive carbon grease, this alone will do marvel’s to keep your Oldtimer in tip top electrical condition.