it should not boil if the tank is vented and there are no external heat sources...65 tank has a non vented cap and a vent pipe up under the fender at the pipe head. its clipped to the body and points down. can see it from the trunk : small 1/4 tube that can be used as an air source or cap it. if you can (low) pressurize it with a bike pump or even a inflated balloon held on the end go under the car with a spray bottle of water and soap and gently squirt the fittings with a stream of this. if a bubble starts to form , there is your leak. I'd start at the rubber hose from the sender.
