I have had several 67/68's. If yours is engineered the same (and I think it is), You will find a mounting point on the side of the interior rocker panel about midway between the front and back of the seat. That's for the retractor. There is a mounting point on the side of the transmission hump, near the back of the seat, for the fixed end.
BE AWARE, if the carpet has been changed, the holes for the seat belts will NOT be cut out. You will have to find them by loosening the carpet so you can see under it in order to locate and cut the holes for the bolts. Also be aware these are hardened shoulder bolts, grade 5 or higher, and can be bought just about anywhere bolts are sold.
Hope that helps,