Definitely the B3 reverse clutch plates.
My experience has been that these plates will wear down to nothing, and the other clutches will still look new.
These plates are really easy to get to (if the tranny is on the bench of course). They are the first thing to come out after the front pump is removed.
Lots of opinions here on going to all the trouble of pulling a tranny and just fixing one thing, but if the tranny is working flawlessly everywhere else, and you can do the work yourself, it might be the way to go. I did just that job a couple years ago, and so far so good.

DG