Beautiful quilt. I have also done a king on my Janome 6600. It wasn't bad at all.
Jansquiltn, I bet those shoulders are all tensed up. Been there, done that. When you are quilting, stop and check out those shoulders, take a few deep breaths and make them go down where they are supposed to be. I get up and stretch every 1/2 hour and sometimes I go somewhere and sit and relax. (My porch swing, the recliner with the laptop, etc.) It really helps.