What if you put a small border on both sides of the quilt? I think you would find it easier if you made the back a few inches larger than the front. You could put marks on the inside of the overhanging backing to help you line up the blocks on the front. It's nearly impossible to align perfectly, but if you are careful, you might be able to come close enough. How are you basting? I think spray baste would hold it more where you want it than pin basting.