Your gauge problems, across the board, are common to many classic cars. Ford began using the 6-volt system, then changed to 12-volts in 1956.
In 1957, Ford quickly went back to 6-volt and kept all their cars and trucks that way through the decades to follow. So... you may have a constant voltage regulator failure. No big deal and they don't break the bank. Be sure you disconnect your battery lead before changing.
Another bigger issue is rust and corrosion, which ruins a good electrical bond. I'm sure you've seen older cars at the light... two running lights are on until the brakes are applied, then one running light shuts off and another brake light comes on, then it goes back. These are caused by bad grounds.
After sitting under two straps for fifty years, your fuel tank needs a copper ground wire to accurately complete its electrical path and display a good gauge reading. One of our members just installed a new fuel tank last weekend, and he included a ground wire. I suggest you do the same:
Read NYsquarebird58's posts HERE