Dave, I've had a good outcome using Gasgacinch on the screws and cork gasket of the sender and teflon tape on pipe threads. To keep teflon tape from getting into the fuel system a couple of bare threads can be left on the small end of the male part so that the teflon won't be cut by the threads and fall into the tank.
My '71 Dodge van has a later year 36 gallon tank on it and the sender is located on the front of the tank, not the top. That's the true test of whether there are leaks. Cork is a low tech gasket material but it sure seems to work in that application. Cork also works great on steel sandwich valve cover gaskets and intake manifold front and back ends that seal against V8 blocks. Good stuff!
There is also a Permatex anaerobic sealer that is not designed for gasoline applications but works perfectly in that application. I used it on the float bowl cover paper gasket on my '36 Chevy pickup that was a chronic leaker. That may be what Eureka Jim is referring to.
