If your backing is exactly x" larger on all sides than the batting (you can add muslin borders beyond the actual quilt back, just make sure the back is centered accurately), and the batting is exactly x" larger on all sides than the top, then you should be able to align the back with the front by making sure the excess batting and backing both stay at that x" measurement on all four sides. Dense basting helps immensely. Disclaimer: I've never done this technique larger than a 60" square lap quilt, but it did work out very well in that case.