Because of the crack's location and severity, we'll need to get access to the underside of the fender. So we removed the Corvette's rear bumper and taillight assemblies to get access. This ended up being a good thing because we found leaky old fuel lines back there!
While we were removing the rear end of the car, we also used our D-A sander to scrub off the paint around our crack, and we found that the crack had been covered over with Bondo and paint before, and it had flexed enough to create another crack down at the wheel arch.
Note that when you use a D-A or any sander or grinder on fiberglass, you absolutely have to be careful to respect the creases and cut-lines in the car's bodywork. If you grind down a convex crease, you'll need to rebuild it with filler and carefully reshape it - and it's far easier to just be careful around those features!