Headrests on 64 barracuda/valiant seats

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I will assume you are talking about bucket seats. Through 66 model the top piece of that seat back is a round tubing. It wont accept headrests. In 67 that top piece was reformed to square with 2 holes for headrests posts punched through.
This change created about 3/4 inch difference in the overall height of the seat back and skin. This should give you some idea what work will be required.
 
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