What's broke?

You must drop the cluster gear, the bearing will not clear the teeth. The arbor tool is cheap. If you have one, it will keep everything in the cluster gear in position while you drop it down and bring it back up. Use a new cluster shaft and key. If your reverse arm is leaking fluid, now is the time to clean up the bore and install a new O-Ring. Same for the side cover arms. It is better to inspect and clean everything and know you are good. Use GL4 lube. I can always tell when a trans was run with ATF just by looking at the worn parts. The cluster gear rollers and thrust bearings stay put with ordinary grease, no big deal.