Definitely check the ground wire. My '51 has one that leads forward (don't know exactly where it went, but I plan on landing it somewhere forward on the firewall (maybe under a screw for the brake switch or dimmer switch.) A bad ground adds resistance to the circuit and can affect what the gas gauge indicates for level, throwing it off. Also check if your cab (or wherever the ground wire lands) has a good ground path back to the battery.
