headliner bow locations

I'm replacing my hardboard headliner w/ a fabric one on my '72. Where do the bows mount? There's 3 sets of 3 holes spaced 9" apart along the side of the roof. The bows seem to fit good in one of these holes. However, there's only 3 sets of these holes per side. Where do the rear 2 mount? There's 2 square indentions in the roof that line up w/ the 9" spacing. Do I need to drill holes in these for the other 2 bows? I don't have my liner yet so I'm not sure if 9" is the correct spacing between the bows.

Thanks for the help.