The easiest way to go is to buy a 72 style rear bumper with the slots along with a set of 70-72 style bumper brackets (used). Then order the rear bumper guards and fillers for a 70-73 . It will all fit like a glove. Just don't re-install the filler piece that goes between the bumper and rear taillight panel.
As for using the 73-74 style bumper, you can fill the extra holes with additional bumper bolts and have a body shop weld up the openings in the rear valance. Unfortunately, the rear guards usually leave marks in the bumper so they usually have to be rechromed. I've seen guys just alter the rear 73-74 style brackets but it isn't easy. Too many angles and everything has to be tack welded, cut, tack welded, etc. Lots of work.
What I did on the front was to remove the heavy reinforcement, remove the filler piece from the bottom of the grille and notch the bumper bracket to frame holes so that it could be slid straight back about an inch. Then I opened up a couple of the holes in the brackets that no longer lined up with the bumper bolt holes and filled in the extra holes in the bumper with bumper bolts.
No bumper brackets interchange between the cuda and the chal.