The lights are not mounted to the grill itself, but to a support behind the grill.
By support they mean crash bar. It's like 4' long and attaches to the frame rails. There are two cutouts for the lights to mount and then the bumper cover goes over it.https://www.firebirdcentral.com/1979_1981_Front_Bumper_Reinforcement_Impact_Bar_p/bum-2601.htm