Help with 70 Dart rear seat belts

So my kids haven't been able to ride in my Dart since I bought it last Nov because it hasn't had rear sealt belts. Today I pulled out the seats and have no idea where the seat belts would bolt. I was expecting to find holes for bolts to go through but nothing. I searched the site for some info about this but did not find anything to help. In the picture the only bolts on the rear firewall are the exhaust hanger bolts (circled in red). Can someone give me some info on this, my kids what to ride in the car bad!