Identification and help

[Barbotte]

As some of you might know already, so for the others I purchased a 1973 Roadrunner project in boxes. Need some identification and info. So, big steel brackets that I can't find in my manual where it goes if it does belong to the car. Second, picture of the diagram for the rear bumper, the reinforcement bracket between the rear bumper and brackets. Did not come with the project. Is it necessary or do I need it.
Thanks in advance