About that "detachable viewfinder: I have a Linhof Universal Finder that I use in the same way you do with my Sinar P2: you find the shots with it away from the camera and then make the shot.

I looked at ebay if there were any GX617's..... they are expensive !
So it will be a 6x17 back for me and make an insert for my Linhof finder.

Have a great one !