I have never seen a 6x8 back in person. I assume they will mount, however, given my previous sentence, they may not due to a physical obstruction of some kind. However, I do not believe that the "gate" of the RZ is 8 cm wide! (72 mm seems to be popping into my head.) Therefore, the only thing using a 6x9 back on a RZ will do for you is to give you one less frame per roll of 120 (and maybe a few extra millimeters per shot).