Replacing gasket on R60/6?

[58fleetside]

The gas cap gasket on my '74 R60/6 is hard and as a result gas has leaked onto the tank and peeled the paint. The top edge of the tank seems to be in great shape with no dents or bends. What is the procedure for replacing the gasket? Will a new gasket stretch on over the metal plate of the gas cap?

I am looking to repaint the tank, fenders and covers so I don't want the same damage to show up later.

Thanks