The struggle is real and i feel your pain, I have 2 cars a 68 Roadrunner and a 73 Duster and both gauges were off by 50% or so. both have new tanks, senders and PC boards in the dash with solid state regulators. Both senders a Right stuff Detailing from Summit IIRC. What I did was to drop the tank and pull the sender. I set the tank on a cart to approximate the same level as in the car, My lift made it easier. Then I jumper wired every thing with a good ground of course. Then I pull the sender out and bent the arm ever so slightly and reinstall it on the cart. Had to fiddle with it about 4-5 times to get what I approximated was 1/2 tank of fuel. I am happy with the outcome and have not had to guess at the level anymore. Before, reading empty it would only take 9-10 gallons on a 19 gallon tank on the 68. Those ground Jumpers are made of the same Junk as the sender sealing rings. I did the same as you did for the ground strap. Kudos to that. This has been an issue with repop senders forever.