I recently replaced just the gasket on the sender assembly. Got the gasket from RD. My old gasket was quite a bit thinner than the one I got from RD, probably from being in place for many years. Before installing the new gasket/sender assembly I cleaned up both the tank surface and the sender surface.
I made a special tool to remove the sender unit locking ring from a black PVC end cap (2 3/8" ID) I found at Home Depot. I ground three small slots equidistant around the edge which captured the ears on the locking ring. Don't try to use a screwdriver or punch to remove the locking ring, that method will likely bend the ears.
I also used a small amount of a fuel resistant sealant on the gasket. The sealant is VersaChem Gasket Sealant Type 2. I think I got it at a local O'Reilly Autoparts store. The tanks are back in the car but have not been filled with fuel so I can't comment on any leaks.
Larry