My best advise is to use the seams as a rough guide, but make sure the bows are aranged so as to give good head room ect. test fit with tape, when its "right" put on REAL seat belt webbing.Its stronger, and lasts longer than sin. You will need a small nail and a butane lighter to "snell" the holes for the retaining screws.
Good luck !!
MD(mad dog)