Catalina 350 International Association

I want to add an overboard fish zinc to my system. I would like to attach it with the alligator clip provided to a backstay but don't think the backstay is grounded. I have made several attempts to find a nut to screw onto to the bolt holding the backstay in place but cant find the size (SAE or Metric) or thread pattern. My thought was to attach a 12 gauge ground wire to the bolt and then bring the wire to the side of the motor where another green ground wire is connected. Can anyone help me solve this puzzle? Does anyone have a better idea or location for attachment of the fish zinc?

Pretty easy to verify whether the back stay is grounded:

Run a wire temporary from engine ground through the aft cabin overhead port/window to near the back stay. Use an ohm meter to verify-- continuity or open circuit.

Alternate solution is run the cable - in my case insulated wire rope - into either of the lockers next to the swim
Platform and through the areas in the bottom of the boat to the engine, and just clip it there.
The locker lid will still latch just fine.
Easy peasy.