If you mean how would you install the stainless steel factory grille, you would only need to utilize the factory mounting holes in the fiberglass, which should already be there……I’m pretty sure every ‘83 Avanti left the factory with that stainless steel grille.
If your car has had bodywork and the mounting holes for the grille were not drilled, it would be easy to drill the required grille mounting holes in the fiberglass.