My 64 does not have a gasket for this panel(7 screws hold it on). Just metal to metal.
On the subject of heaters I have a post in the interiors section asking about gaskets, to date no replies. What this means is that I only know of two gaskets, long thin rectangle one where the heater core box assembly bolts to the firewall (5 screws holding this portion on) and a smaller square one on the inside of of the firewall where the heater duct work attaches to the fire wall (3 screws).