As far as I know the bumpers from 72-74 are the same. 73 and 74 may have an extra hole drilled in it for the bumper guards but they all have the holes for the newer style jack. If you are installing 70 bumperets in place of the 73-74 bumper guards, you just have to fill the unused hole with an extra bumper bolt.
The 74 bumper uses a different bracket to mount it to the body but the accual bumper I believe is the same. Just the bracket and the inner structure that bolts to the bumper is different.
You can use 72 brackets to move the bumpers in. You can also cut the origonals, and then lap the joint and re-weld. As far as moving in the fronts...you can drill out the square holes in the brackets to 5/8" and be done.
One more thing. On the rear, if you move the bumper in...make sure to remove the triangular lisense plate bracket on the rear valence. That bracket was used to space the plate out farther on 73-74 cars.