The holes in the frame, i.e.--the "bumper mounts", are the same. The bumper brackets you need are 70-72 style, which are shorter. Or you can have your bodyman shorten & re-weld your existing brackets. Personally, I think that using the 70-72 mounts is easier. You will also need the 70-72 "filler", a narrow strip of sheet metal that fills the gap between the bumper & body. It's barely noticeable, but you will defininatly notice if it's not there. The filler is being re-popped.

Also, have the bodyman remove the upside down "V" bracket that the license plate bolts to. The plate will now bolt directly to the rear valance. If not, the plate sticks out too far and the license plate light won't illuminate it properly.
You will also need the 2 vertical "bumperettes" that bolt to and hang down off the bumper and fill the rectangular holes in the valance.

There are a few differences in the bumpers themselves. Physically, they are all the same shape, with some "hole" variations, and different style of bumper jacks used. On my 73, I used a 70-71 style bumper, w/o the jack slots. If you do this, you will need a different jack, with a Challenger-specific #102 jack hook. I believe you can re-use your 74 bumpers, but will be left with extra holes where the huge 74 bumpers guards were bolted. You can fill those with extra bumper bolts.

On the front of my 73, I used 70-72 brackets. I have since found out that it's easy to re-use your existing brackets, and re-drill them. You will also need the 70-72 rubber "wedges" for the bumper corners.

Hope this helps... PM me if you have any questions.