Use 3M Strip Calk - #08578 (black). Used to seal the AC Evaporator box halves and also where it contacts the firewall. DO NOT use windshield butyl roll material.
X2 on the 3m strip calk. Why not use the windshield stuff? Seems like the same stuff to me. Just curious why you say that.
I used this stuff on my 67 which required it to be laid on heavy. It has the exact same appearance as the factory. Not sure how this will match the later cars, but where it shows its a good match for the early cars. https://www.v8buick.com/index.php?threads/replace-heater-core-1967-special-with-ac.352573/