[Longhorn Man]

flow direction won't fix a thing.
You need to grab the wife's turkey thermomter and shove it down the defroster vent, and see what the air temp is when the truck is fully warmed up.
Low air temp;
clogged heater core, eithyer internally or ecternally... new ones are cheap.
control assembly not letting the temp selector move all the way.
Heater 'door' inside box is not working properly, damaged bad seals.. ect.
Low coolant temp. If your fans are on all the time, then you need to finish your conversion and get a temp controlled switch in there. your fan motors will not last long being on all the time.

High heater air temp....cab still cold;
cab corners/floor has holes, letting in bitt loads of heat. Plug the holes with tape or rags ow what ever.
Look for holes up on the firewall... like the clutch rod hole.
door seals or poorly mounted doors.