I have a '65 T-Bird and I know what you're talking about. When freshening up some interior surfaces I pondered filling in some holes drilled in the lower passenger side dash, which has a similar kind of textured finish. I decided not to do it because it would've been too difficult to match the texture of the filled in holes with the rest of the dash.
My research indicated some people take a mold of the existing texture and use it on the filled surface, but I don't remember the particulars. It seemed like a long shot so I didn't do it.
Two places to investigate further, if you haven't already:
- The VTCI '64-'66 forum.
- Pat Wilson Thunderbird.
Google and you should find both of them.