Have you looked at UPC C60 Sheets A8 and A10 in the 69 AIM? I'm not familiar with the 69 ductwork and center vent. Is that the original factory duct that came out of the car, or was the dash replaced? There might be better/additional illustrations in the 69 chassis service manual.
Here's a link to an old thread that mentions a seal between the duct and dash
https://www.camaros.org/forum/index.php?topic=770.msg4205#msg4205
Here's a link to an old thread that mentions a seal between the duct and dash
https://www.camaros.org/forum/index.php?topic=770.msg4205#msg4205
