i'm doing something similar to attach the unquilted top to the already quilted back of the gray and pink quilt i'm working on now. the main difference is that i'm using teeny tiny stitches, only in the ditches, and spacing them about 1/2" to 1" apart along those seams. i'm stitching along all seams, both horizontal and vertical. from the front, unless you get up as close as i am while working on it, you can't tell it's quilted at all.