So I got bored and did some more research. It looks like the 454SS rear brake parts are pretty much identical to the standard JB5 rear brakes on an 8.5" 10 bolt C1500. Same drums and backing plates. Here's some diagrams with part numbers.
'91 C1500 454SS:
https://www.gmpartsdirect.com/auto-...-4l-v8-gas-engine/brakes-cat/rear-brakes-scat
'97 C1500 4.3L:
https://www.gmpartsdirect.com/auto-...-3l-v6-gas-engine/brakes-cat/rear-brakes-scat
EDIT - Also just looked at the Little Shop Mfg. site and their five lug rear disc kit is listed as compatible with 454SS trucks as well.
https://www.littleshopmfg.com/gm-88-00-c1500-rear-disc-brakes-5-lug-10-drum/
'91 C1500 454SS:
https://www.gmpartsdirect.com/auto-...-4l-v8-gas-engine/brakes-cat/rear-brakes-scat
'97 C1500 4.3L:
https://www.gmpartsdirect.com/auto-...-3l-v6-gas-engine/brakes-cat/rear-brakes-scat
EDIT - Also just looked at the Little Shop Mfg. site and their five lug rear disc kit is listed as compatible with 454SS trucks as well.
https://www.littleshopmfg.com/gm-88-00-c1500-rear-disc-brakes-5-lug-10-drum/