Bought a set of tubular upper control arms of a member here and when I pulled the balljoints out I found one had been badly crossthreaded. Anyone know what size that thread would be so I can clean the mess up?

I don't believe they correspond to any actual thread size. My understanding is that the factory UCA were made without threads and that the ball joints were screwed in, in much the same way as a self tapping screw or bolt is screwed into any piece of metal, plastic or whatever. As for how to fix your problem I am not really sure. Maybe you can run a hone lightly thru the damaged area. Just enough to smooth the ridges and then try to reinstall the ball joint. I would use some anti-sieze to try to prevent galling. Do your best to get it started straight and torque it to a minimum of 125 ft.lbs. I usually go for 150.

i think that's correct, the new ball joint should cut new threads as it's being screwed in.