Also, when shopping for a Polaroid back for a medium-format camera, take careful note of how large the image will be. For instance, although I have a Polaroid back for my Mamiya 645 Pro, when I use it the image is only as large as a 645 (i.e., 4.5 x 6 cm) negative, which is nowhere near the size of the instant film.

That might be okay for the purposes of testing exposure, but if you're looking to use pack film with a medium-format camera, you might be quite disappointed. It might be better to just use a Polaroid pack camera.