Pretty positive the lower ball joint is the same and it's only the upperthats different form the early disks to the lbp disks. Never once heard anything about changing a lower ball joint. And never had to change the lower ball joint.
When converting my '66 Dart to '73-'76 A-body disc spindles, I did have to change out the lower ball joints, but the '73-'76 lower ball joints bolt right into the '66 lower control arms. The bolts holding the spindles to the ball joints aren't the same diameter, either, so I needed new bolts.
