The problem is that the 73 bumper sits physically lower and the fender indentations for the 73 won't allow a chrome bumper to sit in the proper location.
It can be done, but to do it right requires replacing both front fenders, the grill, the hood latch release, the lower valance and the brackets for the bumper. You will also need 71-72 Turn signal housings and lenses. If using a Mach 1 grill you'll also have to do some minor wiring changes to accommodate the running lights.
There are some people who have used the chrome bumper on a 73 by using 71-72 brackets and then cutting and sectioning the brackets to drop the bumper down to the lower level of the 73 Bumper. I tried this with mixed success and may try again in the future as I have a new approach in mind.
Currently I have aluminum supports and a fiberglass 73 front bumper on mine with a 71-72 grill. This has removed the bulk of the 73 bumpers weight
