Does anyone know where to find a replacement sending unit lock ring for the plastic midship fuel tanks? It's a 1978 F250 4x4. The lock ring is threaded and twists onto the gas tank and is supposed to hold the sending unit in place. I took mine off to replace the sending unit, but now I can't get it to lock properly. Anytime I fill the tank more than half full, it leaks. I've tried really getting it tight, but still no luck. I want to try putting on a new one. Anyone know where to find these? Can't get a very good picture of mine, but I found this in an old thread that looks exacly like mine:

 

It's been a while since I've had one apart but I seem to remember a flat O ring that seals the sender to the tank.