My idea was unhook the ground to see if it changes, I know on the older stuff, the sending unit was a variable ground, from the gauge, there usally was a ground wire from sending unit to frame of the truck, this was pre electric pumps in gas tanks .
that plug you talk about , just unplug that and see what happens, if you can figure out which wire is the fuel gauge feed, with it unplugged , it likely will read one way and ground that wire it will read the other way. ( to the gauge, not the pump.
But if you have a different reading after the bump, I think your float is stuck, and not moving .
think you have to do it again, pita. if you do pull it check it before reinstalling if you don't see a problem with the movement of the float, pull the relay, so the pump don't run with it out of the tank, and make sure you put it in so the float don't hit the side of the tank.
BTW if you still havn't got it working and lift the box, make damn sure to have lots of good blocking, I know a fellow doing some thing like this with one jackall, and the jack slipped out
His mother found him when he did not come in for supper.