fuse box

OK that means you have power up in the cluster, and if it's accurate, you have the voltage limiter working. Here's what is left..............

Poor connections at the fuel gauge in the cluster, poor connection at the rear harness connector in the rear harness, poor connection at the fuel sender or a bad fuel sender, or bad ground at sender.

MOST LIKELY is a bad sender or connections at the sender

So now you get to jack the car up and crawl under there. Do the same thing at the tank sender as you did at the temp sender. The tank sender is right behind the rear axle. Make sure you ground it, and again, turn on the key and watch the gauge.

Did yo download the '72 shop manual?