The '73 up control arms use bigger ball joints. That refers not only to the taper on the shaft, but also to the diameter of the ball joint itself.
So, if you're looking to upgrade to bigger ball joints to do a disk brake swap, you'll need the entire arm. You can keep the upper control arms and ball joints if you use a spacer on the ball joint to use the later spindles with the earlier uppers. Cass (Dr. Diff) was selling a tapered adaptor that you can use to run the later spindles with the small ball joint uppers. Here's a link to his auction...