If you are having gauge cluster problems the first fix is to make sure it is grounded. The ground wire is black and it connects to a post on the emergency brake mechanism. If this ground is missing you will get all kinds of weird gauge and light problems. Trust me - check and fix it right now.
I have the wiring diagram for 74/75 dual tanks, and it sounds like it might be close to the same scheme as your truck.
For a five terminal switch in '74 they used:
18g pink - power from fuse box
18g lt green - power to the solenoid under the truck (to activate it)
18g lt blue - to aux tank sending unit
18g tan/white - to main tank sending unit
18g tan - to fuel gauge in dash
It should be a dual pole, dual throw switch (six terminals). In this switch you have two sets of 3 terminals (dual pole), and they can be switched to connect two ways (dual throw).
- The three terminals on the power wire half will have only two wires (pink and lt green). In main position the power on the center terminal connects fused power to the open terminal (no power to solenoid). In aux position it connects fused power to the solenoid line and activates the solenoid to switch the tank.
- The other set will have the tan in the middle terminal , and the other two sending unit wires on the other two terminals. The wire for the aux tank should be on the same side as the power to the solenoid, so that when you switch to aux tank and activate the solenoid it also switches the gauge to that tank.
You can just leave the switch disconnected and with no power to the solenoid the truck will run on the main tank. In my truck the main tank is the passenger side, but later years they switched it to the driver side tank. To get the dash gauge to read you need to get the tan wire from the gauge cluster and connect it to the sending unit wire from the main tank. If all the wires are under the dash you should be able to make the connection there.