I would just use the foam one. When it compresses, it will fill any gaps caused by differences between the flange and the inner cowl. It should last just fine since it isn't exposed to the outside elements.
I would use 3M weatherstrip adhesive to glue it to the heater flange and leave the other side of the gasket clean in case you ever need to remove the heater later.
Carl


1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission