i use the Horseman 6x9 back and 2 Mamiya RB67 backs on mine. The Mam backs fit fine - especially later ones. I've read that others have had difficulties on their 985s and also VHs with Mam backs, so I think it is all down to your specific camera.
One thing I'd say, if you were thinking of using movements - most of the topcor lenses designed for this camera don't cover enough area to use movements, so you are better off getting 5x4 lenses, and some cheap horseman lens boards from ebay if you are really keen to use movements.