The lower back is for a "Graflex" back with recessed light trap, more commonly found on Graflex reflexes and rarely on Speed/Crown Graphics (usually these latch on with an upper slider that is different from the Graflok sliders). The upper back is the more normal "Graphic" back with a protruding light trap that fits the more common Graflok backs, and standard spring backs if you take the ground glass off.
The "Graflex" roll film back is harder to find as far as I know, so you may wish to make whatever modification you do reversible, or post an ad and see if someone wants to trade you for the normal Graphic back.