Pushbutton Trans leak - stripped bolt?

My pushbutton 904 transmission (1964 Dodge Dart vert, /6, 904, 7.25) has some leaks that I am working on tracking down. One really bad leak is coming from the bolt going into the rear of the transmission above the park cable housing. I tried to turn this bolt to tighten it, but it just spins like it may be stripped or something. I can back the bolt out, however. Since I can't torque the bolt down, the trans fluid just weeps under the bolt head as my car is sitting there.

I can see a flat washer under the bolt head, I assume it's like a copper washer used on a banjo brake line fitting, but I don't know for sure.

Leaking bolt circled here:


I think this bolt is a pivot or something because it does not have external threads that I can see, the shaft is completely smooth as far as I can tell. I can't pull it all the way out because it hits the transmission cross member. I found this picture which seems to indicate it's used for the parking mechanism as a pivot bolt or something.

Bolt circled here as well:


Anyway, I need to figure out what this bolt is and how to stop the leak if anyone has any tips.

Just as a workaround to fix the leak, see if you can back out the bolthead a few turns, and work a tight rubber O-ring over the head and gently tighten down the bolthead again.