I also cleaned up the blower motor switch, gave it a shot of electrical lube, and made sure it moved like it should before I reinstalled it. | 
Next up is the heater core. It's held in the fiberglass "box" by four screws. A 5/16-inch nut driver will be your best friend throughout the rehab. | 
Houston, we have a problem! I can add a blocked heater core to the long list of the heating system's ailments. The new core was worth the money and probably isn't much more than it would be to have the old one boiled out, plus it hasn't been corroding for over three decades. |
Before reinstalling the new heater core, these rectangular rubber seals go on the in and outlets. One is 5/8-inch, and the other is 3/4-inch. | 
This bracket goes around each tank on the heater core and holds it in place, so make sure it fits back on the way it came off. I went ahead and spent the time to really clean out the box before this, and I'm glad I did-there was plenty of dirt and debris in there from the last 35 years! | 
The round seals go on the outside of the box. Again, watch their sizes. |
The blower motor-to-firewall seal needs some good weatherstrip adhesive to hold it to the blower motor box; you don't need any on the firewall side. | 
To seal the heater core side of the box to the firewall, I found this 3M Strip-Calk; there are similar non-hardening products that will work, too. Some come in a roll that looks like rope licorice; this comes in foot-long pieces. | 
You can see the channel that this needs to lay in; make sure to use enough so it can seal, but not too much. Now the whole assembly can go back on the firewall. You may want to attach it with a couple of screws until you're certain the rest of the heating system will go together without a hitch. Don't forget to readjust and tighten the hood hinge if you had to loosen it, and get ready to battle putting in that lower screw unless you took the fender and fenderwell off by now! |