Yup, that's the problem with the '74+ rear bumpers. They're wider than the back of the car, and they don't look right tucked in.
There's a couple of ways to go about converting to the earlier bumper, but either way you will need a '70-73 bumper and the '70-73 rear bumper brackets as well. Once you have those, you can do one of two things. You can make a set of brackets to go from the old shock mount bumper mount on the frame to the earlier bumper brackets, or, you can just weld in a flat plate into the rear panel.
I've actually done both on my '74. The first thing I did was just make up a set of brackets, you can see that install here.
My "new" '74 Duster- or why I need a project like a hole in the head
One side of the earlier bracket mount is already there even on a '74, you just drill out this hole and you have one of the two '70-73 bracket mounts.
The other bolt in the earlier bumper brackets goes right in the middle of the hole in the tail panel where the old shock mount goes. The first time around I made this bracket
This one was welded together from angle stock I had on hand. But really, you don't need to weld to make this bracket. You could just buy some 1/4" flat stock, heat it with a propane torch and bend it into a "L" shape to bridge the space between the early bracket and the shock bumper mount in the frame. There are more pictures and a bit more "how to" in the link I posted.
The other way is just to weld up the "D" shaped pass throughs in the rear panel. I did that later after I decided my car was never going back to stock and I didn't care. That work is here
My "new" '74 Duster- or why I need a project like a hole in the head
This was with the brackets...
This is after I welded it up. I made a copy of the original inside frame bracket, like what you see in the first picture, and welded it in behind this too. I did not use the factory outer brackets that bolt in and have a reinforcement welded to the floor panel.
With either method, you will end up with a gap between the body and the bumper. The '70-'73 cars had a flange on the tail panel that covered this gap.
So with the earlier bumper installed on the '74+ car, you have a gap because the bumper recess is just flat, like this
This is the end piece on a '74+, which is flat
This is from a '70-73. This doesn't have the quarter, but the quarter has a matching lip that gets welded to this one
So eventually on my car I made that flange and welded it in to mimic the earlier corners
And fill the gap.
But even with the gap the '70-'73 bumper looks MUCH better, and you can accomplish that with no welding at all. Just drill out the factory hole through the bracket that's already there, and bend an "L" shape bracket out of some heavy duty flat stock and drill 2 holes in it. Piece of cake!
