If I understand you correctly you need a backing. Unless it is extra wide why did you cut it on the width. You should have cut 2 pieces 12" longer than the length of the quilt, then you would have 6" on the top and bottom for safe quilting. Take the 2 pieces and sew together the length and make this seam the center of the back then you can tear off what you need to from the sides making sure you leave about 6" on each side for safe quilting.
Hope this makes sense.