I never had a problem with the fuel pump but soon I will be converting the signal from analogic to digital(carputer install).
From what I've read there is a floater in the pump that gives the fuel remaining based on the current intensity(resistance). Also, the floater can not be changed on it's own, it comes with the pump and dies with it.
First you should check if the problem is in the gauge itself(maybe the gauge was fried while fiddling with the old/new pump). Best way to do this is get the gauges out and try them on a helpful friends' car. If it's stuck at 3/4 then the gauge is faulty - replace it
Second(if fuel gauge is ok), make sure the wires are connected at the pump(behind the back seat). Maybe spray some contact cleaner or check the wires using a multimeter.
Third(if there is no visible problem with the wires), plug in your old pump. It should display empty on the gauge. If it doesn't then the problem is somewhere at the wires.
Note: In the alfa user manual it says if there would be a problem with the gauge/fuel floater it should display empty and not 3/4 so the floater may be stuck at this level.
Hope my advice helps with the problem
Good luck in fixing it!