Nige, as rackerm stated, the 73 rear bumper guards will physically fit your bumper but will not match the 71-72 front guards. The 71-72 rear bumper guards D1ZZ-17984-A were available separately or in a kit form (D1ZZ-17984-B) from Ford. The kit contained the guards, pads, mounting hardware and installation sketch. There is a company that sells NOS Ford parts that shows stock on the kit.
http://www.greensalescompany.com/ Type D1ZZ-17984-B where it states "Check A Part Number Here". Don't know how proud they might be of this particular part, as like other parts sources with vintage Mustang parts, sometimes they get a Little crazy with their prices. Won't hurt to give them a call as they do show 10 sets of the guard kit. And you can always search for the D1ZZ-17984-A individual guards if their prices hurt your feelings!
Good luck with your project!