I have a 66 that i am trying to change out the upper control arm shafts. Is there a trick to getting the nuts off or has the last 40 years simply rusted them in place?
They're not really threaded from the factory. They just created their own threads as they screwed them in. Expect them to be stripped after you get them out.
There's a pretty nice writeup on Mustang Steve's website for replacing them. The grooves he recommends really work.
I put some together without welding the caps once and it DID unscrew while installed
Luckily I caught it on a routine inspection...
It's been a while since I did mine, but I recall they were a PITA. Best bet is to put it in a vise and heat them up pretty good and then use a bar or something for leverage. If you are lucky, you will not strip the threads, mine ended up being fine.
Do yourself a favor and replace the whole arm(s). They get fatigued with age and it's not worth the hassle of trying to repair them. New arms with the ball joints and bushings aren't that much more in cost.