Few people seem to realize, the S52, as well as many of the other 6s, actually weigh more than the M60. 333 lbs for the S52, vs 321 for the M60.
http://www.bimmerforums.com/engine_faq/
Just thought I'd toss that out, as there has been some discussion of weight on this thread.

Sweet sounding swap, I'm sure it will feel like a different car when you're done. We will, of course, need lots of pictures.