Since we are NOT referring to highly reclined single open wheel seating, it has to be noted, any seat strap has to be restrained at each end to be effective.
Most comments here are focusing on only one end of the shoulder belt, the downside of improper positioning is spine compression, not good.

But what I see overlooked, the other end of the shoulder belt attaches to the hopefully across the hips, lap belt.

When the upper torso in an accident moves forward, and applies tension to shoulder harness, the end attached to the lap belt pulls the belt up into the stomach area. A lap belt across the hips is properly containing the largest bone structure in the body. A lap belt that crosses the lower stomach area is in one of the most vulnerable/softest parts of the human body. The misnamed 5th "submarine belt" in our upright seating sedan cars is what prevents the shoulder harness from pulling up the lap belt into the kidneys,liver, pancreas, bladder, etc. in an accident.
Your gonads are of minor concern here. Positioning well the 5th belt is nearly as important as the shoulder belt mounts.

If your concern is the old factory belts , Why not just replace the OEM unit with an aftermarket 5 point set and only use the lap belt ?? That will give you as good if not better safety than the original one and not mess with your car .

You're only going 80. But in Stock, you can have guys in the other lane going over 160, on 9" tires, and on the brakes.

Just saying.

A little different than the street class at the local track.