These trucks were designed so that a single tank truck could have an aux tank added by the dealer, so the electrical lines to the sending unit should have pop-apart connectors at several locations that are easy to reach (my connectors are round).
For example, on my truck the sending unit has a round post connector for the power wire and a spade connector for the ground. Both wires thread through a grommet on the frame and end up inside the frame where you can reach them. The ground wire is grounded to the frame, and the power wire has a pop-apart connector. When they added an aux tank they just pulled that connector apart and added the wires running to the tank switch, so that it would switch between left/right sending units when you switched tanks.
My sending unit wire to the gauge and primary tank is tan, and the one to the aux tank is tan with a white stripe. I don't know if they continued to use that color code in later years. Some of the wiring is confusing because the dash switch has to be elecrically in between the two tanks.
Bruce