Forest, now this part is food for thought.
"Having the seat belts in the guides gives the seatbelt proper 3 points that keep the occupant AWAY from the side of the car and securely IN the seat in the event of an accident. Having it out of the guide will now make the third point outside the seat and thus not do a proper job protecting the occupant in an accident.
I'll take some pics and check it out. We have experience in shoulder belt retro fits in old airplanes so we are not starting from scratch. Made the full live crash test in one such installation.