Sometime this summer I plan to tear into the "air box" (? or what do you call it?) and repair/replace my blower fan, which currently has a steady knock to it. Any advice? Really dreading this, as I heard it was a bear!

Also, when I do this, is there any way to replace the heater core from the front side of the firewall, or do I HAVE to get to it from the dash side? I have an '81...