well thats a weird change!
better price than i was imagining...
86 should still have 12" rear discs at 24mm wide so you could use these brackets (seen in the picture, call to order)
http://www.wagenswest.com/partstore/index.php/disc-brakes/rear-disc-kits/porsche-rear-disk-kit-with-e-brake-custom-oem.html
and adapt this caliper (with the right piston size and disc width, PN available in catalog)
Wilwood High Performance Disc Brakes - Caliper Product Number: 120-6816-RD