Not exactly what you are asking but I cut a hole in the top of the tank and put in an OEM fuel pump and sender. So it was basically starting from the same point as the sender only location. I did modify the sender float length and the low fuel sensor height. Assuming a sender only is the same length as a pump and sender unit, the modified fuel float caused the fuel gauge to read around a half tank for a longer time than above and below half. Something else I did was after installing the sender, I added gasoline from 5 gallon cans to the tank making a note of where the gauge read as I added the gas. It was a bit of work but I then pretty much knew how much the tank had in it at different gauge readings and how much was left when the low fuel light came on.
I thought about and talked to you about making the tank work like a second main, but it's not happening for a while. I will go with the 5gal jerries to get a feel for what's where though.