I once had a guitar teacher that told me my "mechanics" were terrible. He suggested that I put my left foot on a small stool (they actually make these for playing guitar) and actually hold my guitar into my body and at an angle, in other words the headstock would be pointing up towards say the top of a roof. I could not beleive the difference this made in my ability to stretch, and actually play with little to no mistakes, Just a thought. I still play like this and I wear a strap even when I am sitting and try to keep my back as straight as I can. Hope this helps.
Duck