A ‘76 dual tank setup is relatively simple, but you just have to follow the wiring diagram and not try to reengineer it too much.
First get the right fuel valve
If you have a fuel pump with a return line, you need the six port switching valve. The 3 port valve is used if you do not have a return line on your fuel pump.
The old 1976 six port valves with a single electrical terminal are not produced new anymore, so some use the newer six terminal valve. There are wiring diagrams and project descriptions out there to follow.
Then work the original wiring
With one tank in 1976, the factory routed the gauge feed through the firewall, across to the passenger side of the engine compartment, then down the passenger frame rail to the sending unit at the gas tank. Simple.
When they added the 2nd tank they left all the single tank wiring in place.
- added a dual pole, dual throw dash switch, to switch the gas feed and the gauge at the same time
Five wires at the switch are
DPDT Fuel switch side has 12 volt power from fuse panel, 12 volt feed to the switching valve (and one terminal is open) . Switch just applies 12 volts to the valve so it will switch tanks.
Other side of DPDT switch - wire to gauge on middle terminal, wire from left tank, wire from right tank. It switches the gauge feed for the tanks.
Where it gets complicated is the long wiring attached to the switch.
- one wire runs over to ignition-on 12 volts
- four wires run through the firewall and down the drivers side frame rail
One sending unit wire drops out and connects to the drivers side tank, three wires run across under the cab and over to the other tank.
At the passenger side one wire goes to the tank sending unit, one to the wire that runs up the frame rail to the gauge, and one is the 12 volt power to the valve.
I suggest drawing it out in a diagram in the rough shape of the truck. It helps to get a mental picture of how it is all routed.
