There is some additional work that has to be done, depending on how you go about it and whether or not you want it to look factory.
So this is the what the bumper mounting panel looks like on a '74, big hole for the shock mount to pass through
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This is one of the holes that has to be drilled out for the 70-73 bumper bracket. You can also see that the hole is already located, and that hole is in an "L" shaped tab on the frame rail
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Here's the other issue, the '74+ cars used a plastic filler, so there was no "ear" on the quarter to hide the bumper gap like the earlier cars, the plastic filler did that job
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So the quarter is smooth, with screw holes for the filler
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The first time I did this I just made a new bracket that replaced the shock mount, so I didn't fill the holes for the shock mounts and I didn't do anything to the frame or quarter.
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That works, but you get this gap...
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The second time around I welded up the holes in the tail panel and added the inner hole for the 70-73 bumper bracket.
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I also ditched the brackets I made up before and made up an "L" bracket to weld to the end of the frame rail like the factory had on the earlier cars
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And I made up the "ear" that came on the end of the earlier quarters to hide the bumper gap. Again I made this looks somewhat factory, it's not perfect but from the outside it looks close
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Now, how much of that you do is up to you. You can do it like I did the first time around, which actually didn't require any modifications to the car beyond drilling the one hole. The brackets I made used the original shock bumper mounts and I could have re-mounted the shock bumper if I wanted to. Or you could do all of it like I did the last time around, which was to basically make the car look like a '70-'73 car did from the factory with the frame tabs, filled holes and ears added onto the quarter and tail panel. Or some combination of the two. You could skip the frame brackets and just weld up the holes, but then the inner bolt for the bumper mount wouldn't have much support. And the bumper gap can remain unfilled. Or you could put a piece of weatherstripping there. The factory '74 filler won't fit though.