‘66 Barracuda Fuel Gauge Pegged!

You have proven everything is good until the car harness puts 12 volts in the original limiter. My instructions were, isolate the gauge from chassis ground. Move the 12 volt supply from circuit board directly to a substitute regulator. You did not follow my instructions. You did what you saw at the RTE website. you want to open the gauge to interrupt that 12 volts per their instructions or not?
Your original limiter is still getting 12 volts and still attached to the fuel gauge and that path to the sender is the only path to ground that limiter now has, and it will use it. So in simpler terms you are running a regulator and about half to 3 fourths a limiter, depending on how much resistance at the fuel sender. And you shouldn't have needed so much explanation.