My 73 Chevy needs some help

[RPOZ11]

Hereiis my truck...

Here is my issue:
This truck has the dealership add-on gas tank, a boxy steel unit, that resides under the cab/bed area on the drivers side.

This truck uses a toggle switch on the left lower side of the dash and is electric.

The gas guage wont read correctly the amount of fuel left in the tanks.
I took it to the repair shop that had history of working on it and it still has issues now after 3 months of him working on it.

Items replaced:
Later model side saddle tank seen in newer 75-up trucks with a new tank sender
Tank switching unit on the frame in the rear middle of the truck

Does anyone have any knowledge of these systems and could help direct me in fixing this situation?

I am wanting to fix this by reusing the original toggle switch.
The repair shop added an additionl switch so that the gas gauge could actually be read but I have to switch this toggle switch every time I switch the tanks.

Does anyone have any knowledge or experience with these dealership add-on gas tanks, the sending units in these specific tanks, etc etc...?