If I can impose on a forum member or two with a 1967 or 1968 Mustang to do a "sanity check" on that screw in question. It's a simple request, take a 1/4 inch hex socket and a 5/16 inch hex socket and see what size drive the screw that retains the ignition switch retainer is. It's located to the right of the steering column under the dash and directly under the ignition switch. That measurement will give a "clue" as to the screw size.
Thanks,
Jim