I repaired my 71 grill
Classic crack point in the 71 Cuda grills:
Bottom line, that patch is mesh/fiberglss. Its really not the right material to use. It wont take flex or vibration as it cures very stiff. Key at this area is that that point is a high stress and twist area, when installing and after installation: Prep is the key: Drill out both ends of the crack with a VERY small diameter drill, (so it doesnt get longer), then patch over. But even then, stress with cause the fiberglass patch to crack again (ask me how i know)
GM makes plastic repair products. Ill look and see if I have any left in what I used and get the info off the package