Steve, If its not too much trouble... lets look inside the seat backs too. Since those backs are soooo different, it could be that every 68 and later bucket frame has a cross beam welded in where the 67 frame could have this beam bolted in where needed.
The 67 back frame is a adaptation of the 66 frame. All they did was change the upper member from round to square and add the holes for headrest posts. They changed much more of it for the back cover change.
Just thinking that if these headrests were easily added to any 67 bucket, it would have been done and shared here.