I don't believe there was an actual "standard" since I've seen too many original cars with varying locations of front bumper guards! (Minor, but different... Usually no more than an inch or two off.)
Since the line workers had to drill the holes manually, they probably just eyeballed it. But, to answer your question on location, my car (a 68 RS/SS) has it's front bumper bolts mounted 10" from the centerline of the car outward. In other words, measure 10" out from the centerline of the front valence and drill the hole.
I also believe 67's are the same as 68's, but if I'm wrong, someone will correct me...