I'd be surprised if the threaded inserts are missing from the floor on the
camper -- they are there on my kombi, and I just bolted in the rails from
another van (actually, I got the seat and rails from an '82 Westy -- kinda
odd...).

The bolts (and the plastic plugs) are accessible from above, but the real
flooring is corrugated metal.  If you're not down to the body-colored metal
floor, you're not down far enough.  I think by '87, the campers typically had
that raised-square-patterned vinyl flooring -- you'll need to go past this.
On most Vangons, there is another layer of formed foam-padding below the
carpet -- it has a pattern molded into it to fit the corrugations in the
floorboards.  

I don't know if the standard middle two-person seat from a 7-seat van will
fit in a camper with the refrigerator in place -- the one I pulled this seat
from had the refrigerator removed at the time.  It looks promising, though.
My seat is 46" wide, and the space between the refrigerator and the closed
door (on an '80 Westy) is about 48" -- so it might *just* clear.

Anybody know why one of the seat rails is longer than the other?  I think
you'll have to cut the long seat rail -- or get two of the short ones.  

malcolm