Is this a common problem?
When I took my car to the painter, it had been tracked and driven on the road in primer. My painter noticed stress cracks in the paint (not the fiberglass) below the oil cooler intake that he attributed to flexing at speed. And I was able to easily wiggle the body there. He recommended reinforcement there, so I welded in a support from the scrap bin that bolts to the body. Support is welded to my (removable) lower rad support. Rock solid now, no stress cracks in few hundred miles of driving up to triple digits. Free fix to protect the paint job.
Looks 'factory'...I'll admit that the GTM does not!