I've said it many times that GM recycles some RPO codes. If we followed the generic lists blindly, I'd still be wondering where the 18 speed transmission went from my '93 3500, according to the widely-circulated "RPO Master List" from 2003. MT8 is not an 18 speed manual overdrive transmission. It's the 5 speed manual overdrive transmission (early NV4500.)
Following generic lists for RPO G80 may lead you to believe you have a positraction rear differential in your GMT400. No. You have a locker, the Eaton Gov-Lock. Thinking it's a posi would lead you to use the wrong lube additive and potentially cause issues with the Gov-Lock, which requires that you use no additive.
Richard