Temporary fuel tank patch on a vintage Mustang?

Yes there are many different ways to patch a leaky gas tank. A tube of seal all will easily do the trick. You can also take a bar of lava soap and rub it on the leaking area of the gas tank then take a can of spray paint and paint over the soap (no it will not last forever but it will work to solve the problem and most likely last for years if the car is not moving. you could use epoxy or urethane even body putty wold work in a pinch.