1965 dart fuel gauge doesn't read correctly

First point... The duty cycle of a fuel gauge is a lot different from the others. Inside any thermal gauge is a bi-metal beam that bows with heat to move the needle. The heat comes from a resistor wire wound around the beam. That wire had a spiral wound goldish color insulation that is barely visible. In a lot of cases that insulation becomes caked with crud. It limits beam movement much like a cast on a arm. It accumulates heat and overheats the beam too. You can't know if this is your condition without pulling the inst' panel. if you find what looks like black pepper laying in the bottom of that gauge... there's your 1st sign.
Opening the gauge you might reveal a beam bowed at room temperature. The sad part... it can't be cleaned and/or corrected. You would likely break the insulation and short some portion of the wire to the beam. If you find a good bit of black pepper debris in the bottom of the gauge, that short could have happened already.