I have a Fuji GX680 with adapter for Contax 645 backs on it (cause I use C645 too)... The only problem is with WAs since the wider Fuji can go is only 50mm... So in your case, you may consider using a V-back on the Fuji, since Capture group makes a stitching back for V-backs where one can stitch two shots and cover (almost) the whole image area of the Fuji... Another thing with the Fuji, is that you only need a cable in case you have a multishot back instead of having to invest a huge amount on digital lenses and electronic shutters... An additional advantage is that for real wide shots, one can use 120 film with it and the back is revolving. The only issue one has to overcome over view cameras, is that the movements are not geared and that he has to focus with the front standard.
If I didn't need to use both my multishot MFDBs with movements and only cared for single shot, I would probably consider a Cambo Actus with a FF Sony Alpha on it instead of MFDB...