Where I made the mistake was not the foam insulation around the heater core tubes; I actually got that right. Where I messed up was, there's a foam gasket that is supposed to go around the opening in the firewall ,from the inside, before the heater box is installed. That's how I installed this foam seal, only to find out after everything was all nicely routed and buttoned up, that there were instructions on the flip side of the paper I had been looking at.
Like a dope, I never realized that there were instructions with my heater core seal kit and just reviewed the pictures. It looked like the foam seal went on the firewall (engine side) before the blower motor assembly was installed.
I don't know how much of a big deal that is, I just figured you guys would probably take it apart and do it the right way.