i did this with my '84 wolfsburg also..    i cut the swivel base out of a
dead vanagon westy, with my die cutter (36,000 rpm cutoff wheel)  bzzzzz!!!!
 cut right thru the factory welds..     then i positened the base in plase on
my '84 wolfy front pass seat (i didnt want one on the drivers seat, because
it makes you sit up a little taller, and im already 6ft tall, i dont need to
be sitting any taller) and i welded it in place, where the factory welds
were..   then i trimmed a little bit off the battery box lid..  and  i also
cut out the opening under the front of the seat more, so i could store maps
and junk under there.. , or it is a good place to mount a amp.    i did all
of this with my trusty die cutter :<)   and lined the edges w/ u shaped
rubber edging to cover the sharp edges..  it was simple to do.. with the
right tools.   and to acess the battery, you just swival the seat backwards,
and slide it front..
chris
'84 wolfy w/ passenger swival seat as well as other mods...