SM420 clutch replacement parts

[BlueDiver]

I have an SM420 in my C-50 and it needs a new clutch. Went to the auto parts store and they said it could be either a 10" or a 10.5" plate.

This will be my first time ever opening up a transmission so having the right parts the first go round would be very helpful for my confidence level. I've got a buddy helping me who has done it a lot in the past but he said its been 14 or so years since the last time he worked on one. I feel we are the blind leading the blind. Youtube and a few kind words from you fellas are sure to get me through.

Looking for help on part numbers and /or any advice. Thanks!