Mike,
For info, google "three point retractable seat belts", then scroll down to "Images for three point retractable seat belts". Almost all of these many sites show retractable belts with the retractable mechanism mounted at or slightly below shoulder height OR, the retractable mechanism is mounted to the floor BUT with a shoulder height harness pivot such that in the case of an accident, the shoulder belt exerts little or no downward pressure on the body. Granted, some installation guides show the belt over the shoulder and directly down behind the seat to the floor. However, most manufacturers and racing belt installation instructions show the shoulder belt either mounted to the upper roll bar or include the shoulder height pivot point for the belt.
Think about the physics of motion in an accident. In a head-on, the car stops abruptly but the body continues to move forward until restrained by the seat or shoulder belt. The seat belt will prevent the body at the waist from moving forward. However, the upper body acts like a hinge at the seat belt restraint and will move forward rapidly until held by the shoulder belt. At this point, the body, now weighing considerably more than what it would at rest, is stopped by the retractable shoulder belt. If this belt is mounted at shoulder height, it will restrain the body with little downward force. However, if the shoulder belt is mounted to the floor directly behind the seat with no upper pivot point, the forward moving body will make the shoulder belt move downward with great force. At the least, this can cause a broken collar bone and at the worst, crush the shoulder and spine.
The above web site and others you can research point out the problem of mounting the shoulder belt directly behind the seat. Now, we model A drivers are faced with a dilemma. Let's face it....we are going to get hurt in a bad accident. I believe lap belts only are better than no belts at all. I also believe installing three point retractable belts with the shoulder belt mounted to the floor behind the seat is better than seat belts alone provided you are not in a really bad front ender. The best configuration IMO is to have three point belts with the shoulder piece mounted high as described to prevent crushing injury in a bad accident.
Each of us has to think this through and make the decision regarding seat belts that is best for us individually and for our loved ones riding with us.
Glen