"Seatbelts help reduce the risk of injury in many types of crashes by helping to properly position occupants, while the seat belt is designed to stretch at a controlled rate to absorb impact energy and reduce the severity of the occupant’s deceleration."
This is why one shouldn't simply alter the seat belt fabric lengths and mounting methods of certificated systems. Nor systems designed by qualified engineers with expertise in the area. For example, well-known EAB kits with quality engineering that have cables attaching the shoulder belts to the mount points. Builders sometimes eliminate the cables and lengthen the belts to the attach points. May or may not be a good idea, I'm not qualified to say. Your smilage may vary.
https://gwrco.com/seatbelts/seat-belts/