I just finished a 120" x 100". I did it in 2 sections leaving 2" unquilted on the side I needed to connect. After getting both sections quilted I sewed the top together, turned the quilt over and overlapped the lining. I basted(on machine) with an off color thread through all layers then turned it back to the top and stitched in the ditch through all layers. This caught most of the lining. What didn't catch I finished by hand then took out the basting. I had quilted the center before taking out the basting. This way I only had to handle the entire quilt to quilt the 4" in the center.
Good Luck