Six cylinder u-joints are quite small, as are the u-bolts that retain
them to differential's yoke. The u-bolts' retaining nuts are 1/4-24
(fine) locknuts, and require a 7/16" box-end wrench to remove.
The one on the left (Armstrong) will not fit over these shallow locknuts. I had to go purchase the one on the right (Gray), which has a thinner wall.
