1963 Dart L6 Heater Core

Bumping an old thread here, in hopes of finding someone who can help me find a heater core for a 1963 Dart GT that's leaking. I see one on classic industries, but no way I'm paying $270 for a heater core. They'll be ripping off someone else on that one. FYI - I have no matches searching O'Reilly. Has anyone made a video of removing the box? I've never done it. Thanks!