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I have had this problem before. (on a 68) I didn't have much luck with resin and glass or duraglass, even clamping it together. It held but it was only a matter of time before it caught something (even lightly) and pulled apart.
I am restoring a 65 that has this problem on the front edge of the pass door. I can HIGHLY recommend Lord Fusor 100EZ. It takes a special twin tube gun to apply it, and the product is a bit expensive, but it is clean and you'll never have to worry about it again.
Since I glued the 65s' door back together, it has caught the front fender and rocker many times in the process of shimming and adjusting the body
(and just by opening the door and not being careful, too). The repair is as strong as it could ever be.
Pretty sure 100EZ is good for regular fiberglass, SMC, and even plastics.