The front frame horn ends are different so bumper mounting will take some creativity. 73-80 bolted right to the frame ends. 81-up had a bracket. If you cut off the last few inches off a 73-80, you could probably graft them onto your frame horns, or even better, to your brackets. Then you could switch back with an OEM pair of brackets. One modified set of brackets with 73-80 frame horns for a 73-80 bumper, another unmodified 81-up set to switch back to an 81-up bumper if you wanted.