"Eyebrow" Stud Mounting Locations on Front Bumper

[wingcar]

Hope someone here can help us out, we're putting together my son's 71 RR and have an "eyebrow" for it. Problem is the bumper we have for it doesn't have the mounting holes drilled in it. They layout of the holes shouldn't be a problem as they can measured from the centerline down each side, the concern we have is how far from the back edge of the bumper to the center of the holes? Any advise or pics would be appreciated. Thanks!