More Gas Gauge

[kdml]

I know, nobody needs another gas gauge thread. I have searched and still can't figure out my gauge problem. For background truck is hybrid '83/87. Drivetrain is all '87 with TBI and 700R with dual tanks. I think the only part '83 is the cab and vin. Gauge is combo tach and gas gauge.

Problem is the gas gauge only stays around a quarter tank no matter which tank or how much gas. This morning drivers tank was showing about quarter tank and passenger tank was showing slightly below quarter tank. When I changed the selector switch in the cab the gauge needle moved between these two measurements. Took the truck to the gas station and filled the drivers tank (took about 11 gallons). Gauge now read slightly more than quarter tank. Filled the passenger tank (only took 1 gallon) and gauge went to about a quarter tank, when switched.

What I have done so far:
1) Selector valve on the frame is about 1 year old. It was leaking so I replaced it. Everything worked after replacing.
2) Checked and cleaned the 2 grounds for the sending units on the frame
3) Inspected all wiring on the frame.
4) Removed switch on dash and inspected wiring. The switch is not original (see pics), but did work when I first bought the truck. I planned on replacing the switch with something more original looking (bought Standard DS455, but the new switch has 5 tabs for wires and my current switch has 4 (see pics), so I am not sure how to wire it)

Anyone care to chime in with next steps? Is there some way to check continuity at different points to try to isolate where the problem is? Also, what's the best method to check to make sure the tanks are actually switching, regardless of what the gas gauge says?

Thanks
Doug