I have seen replacement rear frame rails but Im not for sure as to where they are at. Here are a few places to try, Year One, Classic Industries, J&W Nova, Chevy2only, The Paddock and maybe Goodmark Industries. As for the 72 bumpers being different from the 74's, yep they are. In order to use a 72 on the front of a 74, you will need to find a set of 72 bumper brackets, other than that its a pretty easy transition, now in the rear thats a whole other ballgame. The 72 rear bumper has the cut out for the gas filler neck which is located behind the license plate, the 74's are a different seteup alltogether, if you wanted to change your 74 rear bumper to a 72, you will need to replace the tail panel with a 72 tail panel.