I'm about to install one myself, and took a peek under a 69 Camaro in order to check the correct installation. On that particular car, only the two outer rods were used (still not sure where the center, longer one goes). Also, the car didn't have any lower air dam, so the spoiler was attached to the lip of the valance panel with four smaller sheet metal nuts on the sides and two 5/16 thread clips in the center (the latter using existing holes in the valance).

The outer rods are attached using the bolts for the front inner fender/fender connection.

Oh, btw - Bjorn has one on his car. Tell him to crawl under with a camera (I already have...)