Just developed the second film using the 6x8 back. I have good news and bad news. Good news, negs are equally spaced, back is working fine, and have just done a test with my Mamiya 7 43mm finder - this shows nigh on 100% of what is on the negative at close range i.e. 8-15ft, not sure about infinity. Bad news at f11 there is slight cut off at the corners, at f22 there is definite cut off. The extreme of the image circle may just cover the full width. When I use the back again in vertical mode and shift, I will stop down 1-1.5 stops more than the indicated shift on the ready reckoner indicator on the camera.
Roger, there is some horizontal metal linkage in the camera - probably connected to the viewfinder tilt. This is my main concern of what will interfere with the image in vertical mode.
I would also like the Alpa, but the only second-hand SWA 12 I have seen (film back) did not have the lens I wanted, and it was just under £4K, which with hindsight, now seems reasonable.