Is that fresh air vent on the plenum open ?

see the lever for the fresh air door right near where it says ....screw.

 

I would venture a guess that most of these trucks did not have valves to stop the flow of the coolant. I have seen them on 72 and older, and on some later model trucks with AC.

Dennis brought up a good idea with checking the temps of the heater hoses coming in and out of the core. Out of curiosity, have you tried pulling one of the hoses off (not at the core, you can damage it) and see how much coolant you get rushing from the hose/fitting while the motor is running? If water shoots out of the engine, and not the heater core, hook that connection back up and open up the system at the other end. You should get a very strong flow. Hook up another hose to the engine side, and divert both to a 5 gallon bucket to catch it, so you can re-use it, and to keep the mess down to a minimum. Also, this is an obvious thing, but is your coolant level at the correct level? If the coolant is low, you won't get any circulation. This is a tell tale way of figuring out that you don't have enough coolant, so if you heat works ok for a bit, then stops, you might be low. I am not very familiar with the FE family, but does your engine have the heated EGR/carb plate spacer, or better yet, tell us how your heater hoses are routed.

Does anybody have a coolant flow diagram handy?

 

Blocking off the fresh-air vent opening will be alright as long as you also allow the air from the cab get to where the outside air was entering the heater box.

 

This is what I have for hoses.