I recently rebuilt my non-AC heater box and it wasn't too bad of a project. I would check for cracks around where the vac. actuators are riveted to the box. I repaired the cracks with fiberglass epoxy resin. Definitely recommend the gasket set from detroit muscle as posted by
@3C's & a D?
I had some rust on some of the components from the old core leaking inside the box. I used por15 and it worked very well.
Their was a rubber hose that I believe is used to feed air to cool the mouse cage motor, that was brittle and cracked. I was able to repair it but it may need replacing depending on the condition of it.
Careful with the vent duct work and plastic vents themselves. Especially if you have one that connects to the side near the a-pillar, which draws fresh air in. I had to replace that ducting and it was a bit of a pain to find one on ebay.
Pay close attention to where the vac lines are routed and what color code connects to what actuator. Mine wasn't connected when I bought the car and it was a bit of a puzzle putting it back together and making sure the switches opened the right vents.
Another challenge I had was the cable that connects the dash lever to the heater box. Mine was badly bent and was binding terribly when trying to move the heat control. I was able to bend it back and get it adjusted properly it was just a pain getting it reattached.
Take your time, take more pictures than you think you need and see if you can get a manual downloaded or buy a paper one, super helpful to have a diagram of what it should look like, especially if someone else before you "knew better" and fudged it up.
