'73 bumper brackets on a '76 ?

You’ll have to add the mounting holes to the frame rail if you want to use non shock bumper brackets.

And the hole locations on the bumper are different as well.

Here you go. Non-shock mount brackets use two holes at the front of the rail. The shock mounts use a single mounting hole, much further back. The "74+" label is wrong, part of '74 looked like my rail below
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Some of the '74's got all 3 holes because of a production issue with the shock mounts. You can see all 3 on my '74 Duster
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