Just a thought. You are looking for the decorative Barn hanging for your wedding, AND you want it to be a bed quilt afterwards.
Why not make the large block square as the width of your mattress and put on a temporary binding. Then, after using it as a hanging, use the quilt as you go method to add on the overhang from the sides, top and bottom of a bed quilt.
Two uses for the center block, each suited for its purpose.