Seat Belts
 

My 70 Cutlass S has both a non-retractable shoulder belt and a retractable lap belt. The Shoulder belt is larger and fits the only buckle in the bench seat. The lap belt is the same shape but smaller and has no mating buckle to attach to. It is not the notch type I have seen elsewhere that fits into the shoulder buckle. I have two questions.
1) I have not been able to find a mate for the lap belt. Anyone know where I can get one or do I have to replace it with a set of both the belt and buckle?
2) I see 3 point harnesses but assume this requires drilling a hole for the upper attachment and possibly replacing the retractable lap belt below. How do I get the existing shoulder belt off the car in order to install one of these?

Thanks!