I have the same question. I bought the correct appearing metal ones from Autokrafters (I think) and actually tried to install them today. They are just a little too large for the hole, so I am still looking.
There are 2 types used on 65 (and maybe 66). Ones with the serrated teeth down one side and the other with 2 expansion clips at the head base. I usually just re-use originals or get them off parts cars.
They would fit if you opened up the holes a little. They are the "barbed" style (what Charles described as serrated teeth) and for some reason the nose of the pin is actually a little bulbous - like it is larger OD than the shank. For this reason, the pin won't start in the hole, but if the hole were just a bit larger, it would go right in I am sure. The barbs should hold it OK.
I got two pretty good ones at the junkyard, and they are the other style with the tangs up near the head (like my originals) so I will use them.