I set about compairing my two rear beams today.
-Turbo S12 Mk2 top, Z31 turbo lower. Using various mounting points as datum points they're identical to within +/- 5mm.
I offered up my Z31 arms to the S12 beam and they line-up spot-on. TBH I'm not sure if the arms are turbo or N/A, but they're 5-stud;
BEHOLD my brush painted arms!
I then offered-up the Z31 beam to my S12. I didn't fancy fitting my bushes to the beam incase I culdn't get them out again lol so this was a very rough mock-up;
I won't be using the Z31 beam as the machine work done to it is sloppy as fook but otherwise I see no reason why it wouldn't match up.