70 'cuda ball joint replacement

So I go to replace the uppers today and after I get the cotter pin out of the castellated nut both the nut and the ball joint stud are turning.How do I get the nut off with minimal bloody knuckles and swearing?

Drive a pickle fork in there to wedge it while undoing the nut, or just whizz wheel the nut and stud end off

Or gently tighten the nut then loosen again. You can often stop the turning of the ball joint by doing that, then remove the nut and knock the joint back apart.

Those are all good ideas. It's cold and dark now so I'll try it in the morning.

Tried jamming the fork in there, only made the nut harder to turn and the stud kept spinning with it. Had to cut the nut off with a Dremmel . Why do I keep working on these 40+ year old Cars???