factory rear shoulder belts

It's hard to see the detail on those brackets in the pic , but I doubt they are related to the rear shoulder belts. If you look at your sail panel , right in front and a little above of the light bezel there is a small dimple in the panel. That is where the hole would go to bolt the shoulder belt to the C-pillar. Behind that spot is a seat belt boss. If you have a nothback or a Dart then the shoulder belt is mounted through the package tray and secured with a
3-4" "dogbone" shaped bar and a rubber trim bezel that plugs in the package tray for a finishing touch. If you have an original package tray , flip it over and you will see 2 oblong punch outs right at the leading edge of it. I doubt the repro ones have these as very few people know that Chrysel even offered rear shoulder belts as an option muchless have a car equipped with them. I have a set from a '69 Charger R/T .