I recently picked up an unmolested 67 dart GT.
The rockets do not appear to have ever been off.
I thought from looking at some photos that there was some sort of rubber grommet in the hole before the fingers went in but none of the trim clips had anything like that.
When I go to reinstall my rocket clips I will probably add a screw to each one. The wheel opening screws have a very low profile head and look like they would work well.
On a side note... 7 years ago I reinstalled the drivers side rocker just using the clips spread into the holes and (knock on wood) the trim is still with the car. I was even shy a couple clips.