I forgot something. Above the headliner and glued to the roof is a thin material used for heat reduction. It decomposes after time and can fall apart in chunks. I suppose it can collect in an area and be a place for moisture. If the mildew comes back, you may need to park it in the sun and let it bake. My headliner is original, has waves, has shrunken away from the door trim a little and has a few spots. This is normal for these old trucks.

"Adding CFM to a truck will only help at engine speeds you don't want to use."
"I found there was nothing to gain beyond 400 CFM."