Back in the late 70's I tied all my quilts and believe it or not - the ones I still have are still tied. If you really want to sew it - consider using a wavy stitch back and forth over the ditch. It's easier than trying to stay in the ditch. Is it possible to divide the top into 4 sections? It would be easier to machine quilt in sections, then sew them back together. I did watch a video years ago where a lady did quilting in sections - she left the top in tack, then used sections of batting, so it wasn't all rolled up when she started. I am not explaining it well, but it worked for her. I have not tried it myself.