As for the rear brakes, if you want to upgrade and stay with drums, I would go with the 2.75" wide brakes. You will have to change the backing plate as well as the drum. Those were an option on the 12 bolts as well as the 8.5" 10 bolts. The backing plate bolt pattern is the same on the 8.5 10 bolts up thru '86 on the C10's and thru '91 on the R10/1500's.
On a posi differential, it is hard to detect clutch plate wear visually, unless there is something very obvious. What I would do is see if the wheels will turn opposite of each other. If so, then the posi is quite worn. Yes they can be rebuilt, but the cost of doing so can get close to the cost of a new posi unit. Of course that depends on just what parts that posi will need. The clutch packs alone are about $150.00. I would check with Toms Differentials. He specializes in 12 bolts.
http://www.tomsdifferentials.com/
I would definitely do away with that whimpy carrier bearing. To change over to the HD carrier bearing, you will just need the mounting plate that bolts to the bottom of the trailing arm crossmember. If you're not in a big hurry, I may have one available in a couple weeks. And it wasn't just the C20's that had the HD carrier bearing. It was many, many years before I saw a '63-'66 with the light duty carrier bearing in a C10. It appears to me that the trucks that came with the SM420, typically had the HD drive shaft and carrier bearing.