I'm reinstalling my factory deluxe heater set-up in my '62 Bel Air w/o AC. I used the SoffSeal kit which came with gaskets for most everything except the inner heater box to firewall connection. Did the factory use a gasket for this or seam sealer of some kind? A reference to a gasket or a quality seam sealer would be most appreciated. Trying to keep things "factory"! Thanks in advance.