I found a diagram at '67-72 chevytrucks.com' which might help. I had googeled "GM dual tank wiring" to come up with that. The diagram was for a 79 chev with dual tanks.
I Need To Have A Diagram To Wire A Dual Fuel Tank Changeover Valve To The Dash Switch And The Fuel Gauge On A 1978 K10 Chevy Truck?
