assuming the new sender is good [meaning the old one probably was too], you are not getting a good ground to the tank, the sender is a variable resistance to ground, the screws that hold it in ground it to the tank, and the tank needs a good ground which it might not be if the tank is sitting on the original felt and that's under the straps as well - try adding a wire from under one of the sender screws to the cab back wall, floor, strap hold down bolt, or whatever is handy .... the way to check the sender itself is with it out of the tank and clipped from the base plate to ground, run the float manually and watch the gauge
Bill
