Bumper Help

That is right for the bumper. I don't know what was correct for your car. One of those holes would be threaded for the big bolt. Guys certainly do change them to other style -not sure what is best way to do that. I am sure you could find some options if you search this site
It looks to me like that is a 74-76 bumper for big bumper shocks. 73 was last year for conventional brackets.
I think you will need 73 Dart bumper and brackets.