David,
the film sheath is a solution designed by kodak, in fact, origional film sheaths can still be found often on Ebay, and that is where I got the design for these that I had made. The film emulsion will sit less than 1mm away from the position of the origional glass plate surface. Wether or not this will make a difference in the final sharpness I honestly do not yet know. But I would say that after some test negatives, the results should be obvious. But I think the shim idea might be a good one. I'll check in with the shop tomorrow for some shims just in case. In fact, I could do a test right from the get go, same subject, same focus, one with shims one without.

As I placed my order for my sheet film through Webphota in Berlin, the owner told me that he could supply me with glass plates at just about any size, with a speed equal to ASA 100. I must admit it would be interesting to give a glass plate a try, but for practical reasons, I'm looking to stick with film.