72 Pickup Fuel Gauge Struggle

[elkninja]

Howdy, I am new to this forum. I am restoring a 72 GMC 1/2 ton 4X4 long bed. Recently replaced the speedo, factory tac, oil pressure, and fuel gauge. The reason I didn't replace the entire cluster at once is that the temp and elec gauges looked to be newer than the others. All are fine except the fuel gauge. The old gauge was working fine. When I installed the new gauge (from Counterpart Automotive), it pegs out way past full and stays there, with about 3/4 of a tank of fuel in the factory behind-the-seat tank. When I ground the old gauge with the sending unit wire, it goes to empty. When I do the same thing with the new gauge installed, it goes to about 1/4 tank. I ordered another identical gauge, thinking I was shipped a faulty one originally, and the same thing is going on.
Please help if you've been here before.