for the rear you will need everything from an earier year, brackets, bumper, and the 2 bumper guards. The brackets (ebay)and bumper (rechromed) you can find, but the only problem is that the 2 smaller bumper guards are not repo'ed anymore. The bumper guards on ebay are overpriced and pitted or bent. So if you leave off the bumper guards, you will need to fill in the 2 slots in the vallance. Thats what I did to my 74. Also in case you were wondering, the 73-74 bumper will not bolt up to the 70-72 bracket, with or without that reinforcing steel underplate. 70-72 don't use that underplate.
For the front, I havn't tried it yet, but I have read that others have sloted or drilled new holes in the bumper brackets for the frame rails screws, to slide it in closer.