The 67 GTX does not use any rivets.

The clips are plastic and they have the 4 fingers that push into a hole on the body, then you push a round plastic dowel in the hole to expand the fingers. The butterfly clips go into the back of the moulding, put the stud through the hole in the fender, snap the rocker moulding over the plastic clips described above, install the nut on the butterfly clip. Done.