You know what is super funny? I'm going through this same exact dilemma on my '69 el camino. It seems to be 9/16 thread, but I can't find the nut anywhere. I tried parts stores, ace hardware, and the only thing close was Home Depot, they had an empty box for a while but they don't stock it anymore. I was actually going to try the chevy dealership today, and I saw this thread. How funny is that. I'll let you know if I get it at the chevy dealer.