Fiberglass does not bond to metal forever.
Automotive structural adhesive does.
Just close the gap - if I understand it, you're talking about the outer wheelwell and the quarter panel, where the flange is - and it will seal as much as 1/4" gap. The 3M 8115 & 8116 are excellent products; same stuff either #, one # is the GM approval.
If the car's never going back to OEM stock/restored, you might want to trim the flange back or roll it upward for tire clearance.