Even the "Quadra" 4x5 sheet film back for the RZ tops out at 72x72mm. I would think Mamiya would get every last mm out of a sheet of 4x5 film they possibly could without loss of image quality, square, or not square. Then again, it would be pretty slick to own (one of?) the only 6x8 Mamiya SLR's sporting an RZ mount 50 f/4.5 ULD floating element lens with an electronic shutter, and removable, multi-format revolving film backs. Even if it takes several beat up "organ donor" RZ bodies, a couple of pounds of JB Weld epoxy, and 100+ hours of shop time to get it right......

Voyager