[hatzie]

You've probably discovered that 1/2 of the DPDT dash switch handles switching the gauge wiring on the early trucks like your 1977. The centre wire is the gauge and the angled outside wires are the senders. The other switch half only has 2 wires populated. This half feeds IGN power to the solenoid valve when it's switched to AUX.

When a tank pegs the gauge needle way past full it's usually a broken or missing tank ground. Sometimes it's a damaged sender wire on that side. Very rarely it's a bad sender.

Sender resistance to ground rises as the tank is filled.
0 ohms or a dead short to ground = empty.
90 ohms to ground = full.
Open or very high resistance = way past full.