Hi Gary, I had a 72. The GM service manual had a section on just the body parts. I remember that the seam bonding strips are what hold any two panels together. I was not advanced enough to ever get to do any body work on the car, had too many other issues. After looking thru the "body supplement book" I was a bit overwhelemed by the amount of info in the book.
After working in a boat yard that did boat hull repair I came to be taught some of the techniques used in GRP boats. The bonding strips you could get from Eckler's or Mid-America. Those strips make it easier to put panels together, and make the repaired seam have longevity. However, that will only help across the top of the fender, where the seam strip is. The vertical cut you will make will be another seam for which I don't believe you can buy a ready made seam strip for. Check with Eckler's, et al , maybe any piece of seam strip can be adapted to form a seam anywhere you want to make a seam. I could describe to you how a boat yard would address the problem of repairing a panel if someone with more precise auto-body fiberglass knowledge doesn't come along, just PM or e-mail me.
The ideal is if you know someone with a 68-9-70-1-2 genuine GM service manual set ( 5 books I think) cause they will have the body manual book, I gave mine with the car when I sold it. I know you can get Helm & ask if they still reprint C3 manuals. Sorry for the long non-answer, Tom.
Oh, glass work ain't hard, just messy. However it is easy to do wrong, and it may not fall apart but you will have cracks that will not go away!