I used the cork gaskets on the tank covers, and the rubber gaskets on the fuel senders. In both cases, I used Permatex Aviation Form-a-gasket which is a brown, sticky sealant that sort of partially hardens. I coat both sides of the gasket liberally, as well as the screws. There is a very similar material called Permatex High-tack. It is slightly different, but I have had equally good success with that in the past.
I have always had good success with it on autos; it is fully resistant to oil, fuel, ethanol. It is much easier to remove in the future than Pro Seal would be. And it doesn't leak. Personally, I think this is the best way to do it.
YMMV