My sister gave me a quilt with a backing that was too small also. I did the same as you, adding muslin to the top and bottom. Then I hemmed 2 pieces of muslin, about 18" long and 12" wide. As I advanced the quilt, I pinned these 'side leaders' to the edges of the backing. I matched the edge of the muslin piece to the edge of the backing, pinned about 1/2 inch from the edge, then flipped the muslin over and attached the clamps to the muslin. After I quilted that section, I unpinned the side pieces advanced the quilt and pinned the next section, It took some extra time to do this, but wasn't horribly bad. Used lots of pins to make sure that I had even tension. I also floated the top. I just did an overall meander and was careful at the edges.