Both of your requests were standard options in 67. I also have a 67 convertible currently undergoing resto, and although mine doesn't have these, I have seen them and they are described in the various literatures available. The first is the headrests. Standard option and there are many pics of these being circulated. Not sure if the seat back needs to be replaced; it probably does though. Second is not quite so straightforward. The shoulder belts that were available were only 'kind of' 3-point. The shoulder portion was a second strap and buckle from the standard lap belt (if memory serves me). End result is the same; it just requires two bucklings rather than a single. On the convertible, I'm not really sure where the hard point would be. It had to be there somewhere though since the auto world was experimenting with shoulder belts in a variety of ways in 67. I believe it was the next year, 68, when they were standard (or was that 69). Someone else jump in on this if you know the answer to the hard point question.