Just me, but I think it is hard to make a square quilt into a rectangular quilt without it looking odd.
Why not just add 10 inches both ways and have a 70 inch square quilt?
But if you must, maybe put a row at the top and bottom, then add an equal amount all the way around..
Not sure how it will turn out, but it will be balanced
Nice job, by the way.....congrats on a beautiful quilt