Did it to my 73. Best modification I ever did to the car. As mentioned, you need shorter 70-72 brackets. It's just the 73-74's that protrude. For the rear, you'll also need the narrow metal filler piece that goes behind the bumper and attaches to the body, and the chrome bumperettes that fill the holes where your gigantic bumper guards used to reside. Also need to cut the license plate mounting "X-bracket" from the valance, so that the plate doesn't stick out to far. Plate now mounts directly to the valance.

On the front, you can slot or re-drill the 73-74 brackets if you'd like, to move the bumper in. You'll also need the rubber wedges that fit in the outer ends of the bumper. And possibly a new license plate bracket, which mounts directly to the bumper. Can't remember if your old one works or not.