On the E31 forum that I am on there is always a discussion about using 2 DOHC heads from an M50 motor to go onto a M70/S70 V12. Major complaints for the M/S 70 motors is head design and lack of DOHC.
I think someone has attempted it in Europe. Not sure if it ever was operable though. One head would have to be flipped so the exhaust would go outboard.
Link:
http://www.bimmerforums.com/forum/sh...highlight=dohc LouM...