2004 c5500 kodiak fuel guage problem

I hope someone on here has enough understanding of how the fuel gauge works to she light on my problem. The truck originally had dual fuel tanks. Someone shortened the frame and removed the rear tank. The fuel gauge is stuck on full no mater what. The cluster has been sent and tested and repaired. The wiring diagram shows options for several motor option codes. But the diagram doesn't make sense. Can some of you smart dudes shed some light onto theory of operation looking at the diagram. I have one extra plug for tank that was removed and what I think is a plug for a fuel transfer pump that has been removed. I guess transfer pump was removed when fuel lines were redone. What doesn't make sense to me is in the diagram it looks like the circuit that goes to primary tank sender also powers up a transfer pump. how can that work. The repair I'm trying to make is get the gauge to work now that truck has one fuel tank. truck has the lly engine by the way.

When the front tank drops below a certain amount it commands lift pump to feed front tank out of rear tank. Ecm takes an average of the two level sensors to give you fuel level. How to make it work with a single tank.... maybe try finding the correct resistor to trick the ecm into thinking the rear tank is empty? But I really don't know if that'll work, never tried it.