285/50 R20 are 793mm OA, you should be fine to run these on the rear without any major rubbing. Only on full compression may you get a slight rub at the front of the rear wheel well.

http://www.aulro.com/afvb/l319-disco...ml#post2220005

Double check on the front on full lock particularly to the left as they most likely will rub on the wiring loom that runs along the chassis rail. The loom can be moved up out of the way quite easily with cable ties.