The pillar pads, and then the knee pads were "deleted" from the assembly line as part of "less parts to pay the workers more $$" deal. The Nov '67 strike "forced" the automaker to drop some parts....which I thought were mandatory "padding" as per the 1968 Fed Safety Act???
Bumper guards and (on stock Mustangs) the louvered hood was dropped as a standard item, also as a result of the strike settlement.
You can almost date a '68 Mustang by the type of reflectors, whether it has knee pads, pillar pads, or bumper guards--or not!
They just kept putting on those pads at the factory until they ran out of them--then no more. if they didn't have them--and there are no holes--then I wouldn't add them (for authenticity's sake).
Paul.