I considered doing this very thing, even bought both vehicles, but changed
direction, bought a Westy Syncro.  It was a do-able job, but lots of hours
and I decided I didn't have that many to spare.  The toughest job would be
swapping out the suspension parts.  Lots of work, but again could be done.
It's mainly a bolt-on with some heavy drilling and probably a little
welding in some areas.  Correct me if I'm wrong, but I think Steyr just
took a  vanagon and made the modifications.  I don't think syncros
unibodies were any different from other vanagons at the outset.  I hope
someone does this, I'd like to hear an account of how it went.

I also pondered cutting the frame of a syncro from the rest of the unibody,
doing the same with a Westy, and swapping that way.  It would be a lot
fewer hours to do the complete swap, but riskier.  (What do you do in your
home shop if the unibodies you're trying to mate don't match up very well?)

I think it would be less work to convert the syncro van to a custom camper
of your own design (always wanted a Country-Homes type straight-up poptop
with Westy cabinet design) and perhaps a little more rewarding creatively. 
But, it wouldn't be a  Westy. 

Bill
www.geocities.com/yosemite/gorge/1706