You need both the bumper and the brackets from a 70-72. The 70-72 bracket will not line up with the holes in your bumper (or the reinforcing plate). All the rear bumpers are the same shape and size, but the mounting holes changed on the 73 and 74 ones.
If you want, I guess you could drill your bumper so matches the 70-72 bracket. But then what about the holes from the original bracket? Are you going to paint the bumper? Then you could fill the old holes.
Next problem will be the bumper guards - slots in the rear vallence. You did want to ditch the huge rubber ones right? You will need the 70-72 bumper guards, or fill in the slots in the rear vallence.