Here are some pictures of the heater core case that is inside the car under the passenger side dash. There is a foam rubber seal around the edge of the case to seal it to the firewall. There is also a dense foam seal that goes over the heater core tubes. I haven't been able to find reproductions of these seals anywhere.

The inlet tube is the smaller of the two heater core tubes. The factory inlet tube is crimped down at the open end. This is to restrict the flow somewhat and to prevent a high pressure surge from blowing out the core when the engine is revved.

There are three mounting studs on the heater box that go through the firewall - one next to the heater core inlet tube, and two on the opposite side of the heater box.