I saw the article on installing one on newer Cougars yesterday. Well done. I just happened to be doing the same task on a Cougar. Since no 1967 - 1968 Cougar came with such a mirror it doesn’t belong in a restoration thread. So I am placing it here by itself.
The passenger side mirror is made by Scott Drake. It comes with a gasket and two self tapping screws.
This is not the method I am going to use to install this mirror.
Instead the AMK body kit comes with two #10-24 oval head countersunk screws and two #10-24 rivnuts. That is how the factory mirrors were installed and that is how I plan to install the Scott Drake part.
I started by installing masking tape in the general area were the mirror is going. Then I measured the mirror on the driver side to locate the most forward hole. It was exactly 10" with the tape measure as far up against the wind wing as possible. Then I measured the Scott Drake mirror to achieve center to center on the holes. This allowed me to lay out the hole locations and put marks on the tape where to drill.