New Gas Tank Sending Unit Leaks

I had the same problem with my '67 Barracuda and my son's '70 Roadrunner. The tanks were fine but I had a devil of a time getting a good seal with those new lock rings. On one tank we were able to find and re-use the old ring. - On the other one I put some Permatex AVIATION FORM-A-GASKET #80019 on the rubber seal and forced the locking ring about an eighth of an inch past the rotational stops. I was pretty conservative with the sealant because I wasn't sure if it was soluble in gas. I figured as long as I could get it sandwiched between the tank and the gasket with a thin coat I'd be OK.