Yet another gas tank probelm

I've replaced my sending unit gasket twice (& second gas cap) now on my '75 Dart, but can't get the tank to stop leaking. Seems like whenever the tank is over half full and especially after it's been driven a while the tank will start leaking from the sending unit. I've got the vent line running from the canister to intake vacuum, so it doesn't seem to leak when running. However, once the car is shut down, it will start to leak. Am I supposed to be using any special sealer on the gasket? If so, let me know what type, since everything I've found say's not to use with fuel. I know it's a poor design to place the sending unit below fuel level, but it had to work properly at one time.