I had an issue with my ‘64 ford dump over the summer that was similarly frustrating. Full tank but starving for fuel. It turned out the fuel pickup line that enters the top of the tank has a leak somewhere in the tank (most likely around the fitting?) and it was sucking air. The lower the gas level, the more suction it required, the more air it would draw in. I finally figured it out when I put a clear fuel filter in-line and could see the air being pumped in. I’ve had a gas can strapped in the passengers seat ever since. I’m still trying to figure out how I’m going to address it that doesn’t involve a new tank.