The issue for the upper joint is that the top of the C is flat, whereas the lower part of the upper C is angled. See the lines below. If you used a regular flat symmetrical adapter, the force is transferred unevenly. You have to have some way to support the lower part at a different angle in order to get the upper ball joint to go in straight. Basically, you want the tallest part of the adapter on the outboard-most edge.
View attachment 319072
I built an adapter and it got the job done, but realistically, I need to re-cut it even steeper to work easier. On the very most outboard side, you could stack washers or metal stock to fill the gap, but it's kind of a pain in the ass, I've done it.
View attachment 319073
Sucky way to do it (picture from internet):
View attachment 319075
If you choose to do it this way, you have to make sure the washer/metal bar isn't inserted too deep or the ball joint will hit it while you're pressing it in.