Well, while you have the dash out to replace the heater core take the time to refurb the air box. You're almost guaranteed to find that all the foam seals in, on, and around the various doors are disintegrating into powder. Your A/C and heat system will work a lot better if you pick up some proper thickness foam, cut it to size, and replace what's long gone.
Oh, and just a quick warning. The pins on the back of the dash that fit into holes underneath the windshield are likely to be pretty brittle and will break if you're not careful moving the dash. And they're aren't available from Toyota as replacement parts. I had to get them used from a guy with an already trashed dash.