Piecing the back will be fine. After it is quilted the seam won't be noticeable. Since you have two lengths of fabric you could always cut the backing in equal size pieces with the seam in the middle and use the leftovers for another project. Or if you have enough fabric left over from the front (or even other flannel fabric) you could add a vertical row of blocks down the back that coordinate with the front which would give you the last 6" width you need. I often add some blocks to my backs for interest.