Anyone have a trick on how to keep the shaft of the UCA ball joint from spinning while I try and reinstall the nut?

I whack it into the taper with a hammer and then hit it with an impact gun. If you don't have a impact wrench, it can be a real PITA

Install a clamp to put the pressure onto the UCA ball joint. I think that's what I did.

So the ball joint is tapered and "should" seat itself into the spindle with a few whacks?

It will seat, but not well. It will move with a little upward pressure until it is tight. I usually have the socket on there...whack it, and then run the impact wrench. Sometimes you need to do it a few times.

Found a super easy way to do it...put a jack under the LCA and jacked it up as high as it can go. The angle that the ball joint ends up at creates enough side force to keep the shaft from spinning...easy as pie...Thanks everyone...

i just grab a piece of pipe or something to use as leverage and just push down on the ball joint against a spot on the frame...usually if you get it started enough it will hold itself against the taper