I have to concur with JoeFrazer that problems such as you describe are almost always the sending unit. I had that happen in my Allstate - you'd fill it with gas and the gauge would read empty. Drive for awhile and the gauge would go to half full. Keep driving and it goes empty very quickly. Replaced the sending unit and that fixed it. Had similar problems with my other cars. It was always the sender, never the gauge itself. But of course, with anything 70 years old, it could be the gauge but check the sending unit first. Best of luck and keep us informed.