Stock Seats and Cage/harnesses question

i have a friend that builds off road trucks. i showed up one day in the middle of a rather heated discussion about shoulder blelt placement. i know that rule books require the shoulder belts to be mounted in certain areas. that was not the arguement. one of there customers was an ortrhopedic surgeon. and this customer wanted the belts mount moved form the crossbar to the seat mount on the floor in his prerunner. dont remember the reasoning, something to do with if the seat were to move rearward upon impact, the harness would loosen. if the harness were mounted on the seat mount on the floor, this issue would not be present. there was several other issues, this was just the one i remember. any thoughts?