[PbFut]

Took a few shots of the tank and how the factory set up the auxiliary tank. It is equipped with a fuel gauged sender. Fuel is diverted using a 3 port manual valve. The valve is stacked with a second valve so it will divert both supply and return. This eliminates the issue of the return fuel going into the main tank and over filling. I had a 71 Chevy C20 with aftermarket tanks and this was a problem until I realized you needed to run the main to empty then switch.
As for this OEM tank, here are a few more pics.
After opining up the port for the sender unit, it was clear the inside of the tank is in pretty bad shape. There was a lot of water in the tank from the failed filler hose in the truck bed. I cut two access holes and will try to descale as best I can manually before attempting a chemical etch and seal.
I took a few shots so anyone looking for documentation on how the factory did the option will have some info. I add a second post with a few more. These with the 3 photos I posted above might help someone down the road.