A chamber casting would be in order to establish that the existing neck diameter and the neck diameter of the EDGE reamer are compatible (they are supposed to be, but factory chambers can be on the generous side). If they are, I'd set it back a turn (1/2 turn might do it), as the head sizes are the same diameter and I'd want no shadow of the .338 RUM chamber to be left, and cut the EDGE chamber and mark the barrel for the new chambering.
