To further complicate the issue, I have what *MIGHT* be a positraction rear end in my new '59 that I will need to do brakes on soon.
It's a Spicer 45 rear, which were standard with GMC light duty but not on Chevy's, and the only reason I can see that a Spicer 45 would end up under a Chevy is if the posi was ordered, or the guy asked for a heavy duty rear axle and they sourced a GMC part for him instead.
This parts Wiki lists the rear brake shoes as a differentiated part for 58-59:
http://www.gmpartswiki.com/getpage?pageid=226187
Which is also interesting, because the posi rear was only advertised as a '59 improvement, although maybe late-year 58's were able to special order.