Gas gauge not working staying past full what could it be

Yes the sending unit is mounted with the pump. Look for a broken wire going to the tank or a btoken ground wire, the wires back there are probably pretty brittle and may have broke when you lowered the tank to change the pump

If the above didn't work you may have hit the float when you put it in and they can mess up I had to replace my fuel pump in my car and hit the float when I put it back in and had to pull it again just to replace the float