That is my mistake. After a little bit of research, I realize that I should have said that you don't compose the entire shot if you use the standard 6x7 focusing screens. You'd need to get the 6x8 focusing screen to show the entire view. I believe you also need a 6x8 film back adapter (which also works for 6x7). I do not believe that Pro SDs (with their 8x8 cm rear aperture) have quite as many issues to consider when using the 6x8 back. I will defer to the experts on 6x8 backs to explain the details of their use in detail.
"Truth and love are my law and worship. Form and conscience are my manifestation and guide. Nature and peace are my shelter and companions. Order is my attitude. Beauty and perfection are my attack."