After 20 or more years, the foam used in the heater box turns to mush or is blown out of the box. This leaves the heater control doors/flaps with gaping holes.
One flap simply moves the air either up toward the dash or down to the foot wells. The other one engages the air to flow or not flow through the heater core.
Common practice it seems is, when rebuilding with a new core, to block all of the holes with foam or foil tape.
Question: is there any chance of overheating the core section if air flow is completely or drastically restricted from flowing, at least a little, past it?? (In practice, there seems to be a lot of air leakage)
It appeared that on mine, only some of the holes in the heater core control flap were partially covered when new.
Again, is it ok to cover all of the holes and not worry about the core overheating the heater box and causing a meltdown???
All of the doors should have foam on them... or foil tape.
The heater core is of a free flowing design, meaning that there is no valve that controls coolant flow through the core. Think about how often you drove the car with the heater & blower off. Overheating the core is a non issue. If you leave holes in one or more of the flaps, then your control of air flow (directional up / down, or through the core to get heat) will be affected.