Thank you so much for your responses. For some reason it does my heart good to know there are a few of you who don't like to put king size backs together either.
I did bite the bullet and finished the back by dividing the fabric up in thirds and pinned and sewed it together. I took wider seams than I usually do and trimmed them down to about a 1/2 inch. I think that helped. Thanks, Sewing Pup.
Rhonda, I believe your method would work on fabrics that need matching and maybe a smaller quilt. It sounds a bit more labor intensive but very accurate. I'll keep that in mind for other projects in the wings.
And maybe looking into the wide backs would help me even more. I have not used any wide backing fabric. However, most of you recommended that prospect. Thank you all very much!