When I was flying my first Model IV, I became concerned because the shoulder harness attachments were not centered behind my head - sort of spreading in an aft Y shape to their respective attachment points. I was always concerned I could slither out between the straps in an impact. I made a couple of velcro straps to hold them together across my chest. When we were in our emergency landing (crash), I found that the force on the shoulder straps tore the cross straps apart - 1 inch nylon. I repositioned the attach points to behind my head on the new IV. It required some welding.