Remember, shoulder belts stop your upper torso from going forward only, the seat belts stop you from going forward and up. So I would set them angling back or to the back seat belt holes. Pretty much in same line as with a cross bar behind seat.
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.
