>mount tracks and then cut a strip of wood to lay in them
>when seat is not in use, and recover with floor mat.

Actually, if you find someone parting out an '85 Westy, you may be able to
get the factory metal covers that go over the tracks when the seat is not
installed... this is a pair of three-piece fillers:

         one fills the space between the sliding door and the seat.
         one to fill the space of a single middle seat (left out if the
single seat is installed)
         one to fill the space between the single seat and the cabinets (I
assume this is left out if the double seat is installed).

I don't know if these were provided in earlier years, but I know my '85
came with a set (though it didn't come with the single seat... I had to
find one of those on eBay).

Tim