Hi chris!
I own the same camera with the 4x5 back and the 6,5x8,5 back. It's a great camera, really light. I was looking for a 5x7 back and was suggested not to buy the original 4,5x6,5 back because slightly smaller than the 5x7 standard. So I bought a 5x7 japanese wooden back and adapted it to the camera. I was really interested in having a look at your back's adapting work. I'm working at the moment on a wooden adapter to use a sinar 8x10 metal back on this same camera.
many kind regards from Italy.
paolo