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Ok, I'm racking my brain! I've had several Ford trucks through the years and all had excellent heaters.
1977 F150 w/Fac/AC
I get a weak airflow.
I've had the dash apart several times. All the vacuum switches and heater controls seem to be working. Blower works at all speeds. The various vacuum actuators all direct air to the proper outlets. Core gets hot or cool depending on switch position. Gasket on fan motor box is there and bolts are tight.
I've checked and rechecked for obstructions, but the flow is very weak.
It sounds like you have covered most of the bases. Here is one more. The heater core can get plugged solid with dust, dirt, ect. on the front side where you can't see until the core is removed.
I don't know if this applies to your truck or not, but I have heard that some of the intakes have a screen or filter before the blower that can get clogged. Just a thought.
Good Luck
I had a heater problem on my 79 f150 and it was the valve on the heater hose line in the engine compartment, it was stuck closed, i pulled it off and used wd-40 on it until it freed and just worked it back and fourth and it gets hot now.