Another complication may prevent the cells of the 50ULD from working on the RB67 even if mounted in an RB lens mount with mechanical shutter.
Lens mount to film plane distance is shorter on the RZ than on the RB, meaning that the hybrid will not focus at infinity on the RB.
Unless, that is, the rear element of the 50ULD is recessed from its original RZ mounting ring (as viewed from the back of the lens) AND that the donor RB lens' shutter is the correct distance from the mounting ring. Different focal lengths of lens vary widely in this respect.
6 x 8 is nice, I use those backs on my RB. The main reason I use them is that they're electric wind.
You can crop a 6 x 7 to the same proportions as 6 x 8 with the loss of only a few mm. of film so why destroy the resale value of two lenses?