Hey 42CHEVCOUPE- Welcome to the forum
Perhaps you could find a catalog that lists the specs for different ball joints, and find the ones that match the fit for your chevelle spindles and mustang II control arms.
Maybe the upper control arm could be drilled to fit the larger stud... I don't know if that would cause any structural concerns for that arm though.
Good luck! Lot's of great people and info here- make sure to give the search feature a try. And throw a little punctuation in your posts to help out the guys reading it