Tolerances are not a big issue and the Mamiya 6/7 lenses don't have any focus shift I ever noticed. No, the big issue is you are making a frankencamera that would be a monster to use. My all-time favorite camera was the Mamiya 6. It was small and light, fast and easy to use, and inconspicuous. Everything this modification will not be. I would go for an Alpa TC with a nice Rodenstock. By the time you sink all the money and time into this conversion, you can easily afford the Alpa. And it will be bigger than a TC. Either that, buy a Leica M9, ME, or M, if you want a rangefinder.
But it can be done. But you are making a digital only camera.