The heater box is under the dash so the seal is also on the dash side to seal heater box to firewall. #3963725. There's nothing on the engine side, just the heater core pipes coming through, each with a sponge seal around them as well.
Some sell a reproduction, otherwise just use close-cell neoprene with adhesive backing on 4 sides.
Random ad.
https://www.chicagomusclecarparts.co...amaro-trans-am