Ok Larry What do we know....
We know that the truck runs and it is belching Black Smoke.
We also know from your inspections that fuel is not going anywhere but into the engine.
Black smoke means that there is excess fuel that is being burnt.
(White smoke is fuel that is unburnt)
So from this we can tell that you are having too much fuel injected into the combustion process or it is being injected at the wrong time.
So wha is the cause???
Could be a dieing IP, It could be an IP timing issue, could be a stuck injector/s. Is your cold advance switch working correctly?
Too much fuel can also mean too little air. Is the filter dirty?
Is the inlet blocked, has the rubber elbow kinked?
These are my initial thoughts, but I'll keep thinking.