You need to inspect the rails/brackets on your existing
van.  I don't know the years, but I know that on for instance
my 80 vanagon->adventurewagen, they are welded onto the bodywork
(when the rails were obviously dipped in the pain bath with
the rest of the body that's a telling sign).  Since I can't
believe they made two sets of bodies, I suspect it became a
possibility at a certain point when they changed the fixture.

If your rails ara apparently mounted on a black metal structure
of some sort that attaches to the bodywork with 4 bolts in
about a 9 inch square, then you are in business - these units
should swap straight across with the swivel units.

I found this out, by the way, the hard way; I'd bought the 
car the day before and was rampaging in a junkyard and 
found a more recent model westfalia in much the same state
as your parts car, and pulled the swivels only to find
on returning home (this is why you should always take the 
destination vehicle to the yard if possible) that they
don't fit.  So if anyone in the bay area is in a position
to take these off my hands (swivel structures with rails for
seats) let me know.

art walker
ahw@egret0.stanford.edu