David,
Congratulations on your 5X7. The focus shift problem will be averted if you mount the fresnel at the rear of the ground glass. That should be fairly easily accomplished by removing the GG retainers.

I came across a site at Camerascreens.com that handles 5X7 fresnels and they indicate that they will need the exact measurement of the ground glass or the existing fresnel in order to supply the proper one. I imagine that you should be able to measure the ground glass dimensions without removing the glass or the back.
Additionally I encountered a fellow up in Seattle, I believe, that handled about four different types of fresnel screens. I don't remember his name but his Ebay ID was Omni View. I bought a Dagor lens from him about a year ago. Maybe you can pull something up on him as well. Good luck.