Mounting Seat Belts

I just noticed that the 1965 Canadian Valiant convertable that I restored has never had rear seat belts. I removed some from a 1966 4 door sedan and I see that where the middle belts mount there are holes with plugs in them in my car but I can not find holes for the outside portion of the belts to mount. I don't think they can be located where the 4 door has them because of the fiberglass unit that the convert. has. I thought before I start drilling I would see if anyone can help me locate where they should be located. located.