Im not trying to sound like a smart-ass here, but if you said that the gauge reads 1/2 full when it is empty, then with the tank empty, cant you just pull the needle off the guage and press it back on at the empty line? Then it will read right at empty. It shouldn't really matter where the needle is at when you have a full tank, right? I mean the real intent of a fuel gauge is just to let you know when youre about to run
out of gas.
Definitely keep us posted though. I'm about to undergo the same swap, and I was wondering what to do about the sending unit.