:driving:
The fronts should be easy. On my wife's 73 cuda, I just slotted the bracket holes where the bolts go through to bolt them to the frame rails. I used a torch and then cleaned up the openings. Just eliminate the filler and slide the bumper back 1/2". I think I just used two of the bolts on each side. I removed the thick bumper reinforcement altogether and bolted the bumper directly to the brackets. You will have to open up a couple of the holes in the brackets for the bumper bolts to go through.
The rears can be a little trickier. The best way of course is to locate a '72 bumper setup. The 70-71 wouldn't be correct because of the slots in the bumpers on the 72 up cars. We have used the later style brakcets by cutting them up and bolting the frame halves on the car and the bumper halves on the bumper. You then need to locate the bumper and cut more of the brackets off until the brackets are close. You then have to tack them in place with a mig and when you're happy, remove the whole setup and weld it up. It's difficult but not impossible.
If you're painting the bumpers to match, you can weld up those big bumper guard holes and grind them down, putty them, block, prime and sand prior to painting. They will look great. Hey, if Chip Foose can do it, you can do it.
Good luck
