Rear seatbelts 64-66 barracuda

If complete safety is the issue you really need to address the seat latch FIRST. It's not just the belts that create greater safety and it is possible that changing the belts only will make it less safe. Early Barracudas have a sheetmetal floor that is screwed to the welded floor. Attaching belts to that would NOT be a good idea, in my opinion. Check how some modern cars accomplish this and use those parts or engineer your own.