1968 Airbox rebuild question

Inspector71 said
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I figured all that out with your pictures. I ordered the heater core through Classic Industries several years ago. Fortunately, I saved the other heater core. Based on your advice, I'll dig it up and compare. If the new one is not a match, can you recommend another source? Please forgive my ignorance, but how does one repair fiberglass? As always, I appreciate your patience and help