The choke is not functioning correctly. With this kind of problem it is usually the coke unloader, or the electric heater in the automatic coke control. with the engine cold remove the air cleaner. Depress the foot feed once to set the choke. start the truck, have a friend watch how the choke operates, make sure he stands away form the engine compartment in case there is a back fire. When you depress the foot feet when the engine is cold the choke will close. make sure the step up cam is working this increases the idle speed. When the engine starts the unloader will modulate the coke plate so the engine is not starved for air, as the electric heater for the coke coil operates the choke will slowly open. If you don't see the plate modulating it is the unloader, if you don't see the choke opening slowly as the engine heats up the choke coil needs to be replaced
The one thing I don't know about 1986 carburetor is if it uses a heat tube from he exhaust manifold to bring in hot exhaust from the engine into into the coke coil. If you have one make sure the tube is attached and it is it is not coked shut..
The first thing I would do is get yourself a can or two of spray carburetor cleaner and hose the carb' down and make sure everything is clean, and all of the linkage is working smoothly. this might just do the job.
The single barrel carburetor the 4.9L 6 has is a simple unit to rebuild but If you don't feel comfortable working with carburetors, I would take it to a tune up shop that has the experience and have them do the job.