I bought a sharktooth grille from a kid who had broken the mounting tabs and mounted the incorrect stainless trim on it using screws that mounted in the wrong places and ended up with holes in the outer grille, to top it off, the screws he used rusted and left stains. I used two part epoxy like kamstra had done, and held the mounting tabs pieces together with rubber bands while the epoxy set up, Not too tight!
I then smoothed out the joint and layered on some white putty used for plastic model cars etc. It sets up fast and you can then sand it and shape it just like it was bondo. I used it to fill the holes in the grille where the screws punctured through the front of the grille and it turned out perfect after tons of masking/painting. All those teeth in a sharktooth grille are REALLY a pita to mask and paint. but worth the effort.