Alright. The program I have can let you change pitches of the song so hopefully I can make Tupac rap in the key of E Major (which God Knows is written in) and I guess I could pull it off. If not, then I can use Stranger in Moscow by Michael Jackson which most of the parts is in E Major.
I couldn't even concentrate to figure out the keys.
Personally I would just use 2-pacs monologues over the God Knows... (Instrumental)
the vocal harmonies would be really tricky to incorporate though.
To be honest though, my ears are so accustomed to atonal stuff that it wouldn't bother me if you mixed them, but regardless: you might try at least shifting the pitch of the 2-pac vocal harmonies to mach "God Knows" tonality, if you have the ability to do that.
Anyway, if you end up going through with this project, please send me a link. I'd love to hear it.