This may or may not help, because I have a 69/70 rear wheel without the cush drive. The piece in your picture in your fingers is the top hat washer that goes into the speedo gearbox. It's sort of a spacer. When I put my wheel together, I had the bearing spacer, item 29 reversed (it's different on the 69/70). I don't know if that may be the problem on your newer rear assembly, but it may be worth a look. My problem was the spacer has shoulders, short and long, and it is reverse of what one may think, the double bearing goes on the short shoulder and the single bearing goes on the long shoulder. Again, I'm not sure this has anything to do with your newer rear assembly, but it may be worth a look. When I had mine wrong, I would tighten the axle and everything would bind.
Dave
69S