Since the 850 seats are smaller, I'd say you should not have a problem. At the very worst, you might have to put some aluminum angle stock on the 200's mounting points (spanning them for stability) and then drill that bracket for the 850 seat mounting bolts.

Recently I put a set of 90 240 seats in my Datsun 510 wagon. Spanned the rear pedestals with 1 x 1 angle which I drilled to take the horizontal bolts on the back of the 240 rails (where they have that angle bracket). About a four hour job all told. Up front I just drilled new holes for the rail bolts and plugged the old ones.

I would recommend you take a look at seats from a late 80's to 93 240. These have the deeper, reinforced side support rails under the foam. I re-do 240 seats regularly. Normally my routine is to find two, good, passenger seats from which to build a good set of fronts. The later 240 seats are really very nice. The biggest advantage is that you will be able to bolt them right in. You can get door panels and even back seats to match, and colors will work with your carpet.

Not that we don't love the seats in our 850. However the cloth seats are by far the most durable. Around here, at least, 850's with cloth are very rare. Ours has the "leather faced" seating". It has held up well, but only because we maintain it rigorously. Of our 240's, the cloth definitely holds up better than the "leather", which is supported vinyl in our '83.

DS