FWIW, Mercury Camera Co. makes a 35mm Pano kit to convert the RB 67 back to handle 35mm film. It replaces the film gate of the back with one with a 35mm 24x67 opening. That should help film flatness. They don't address the film counter as well as is discussed here.
There is also a 3D printable version of this here, in case you can 3D print stuff.