This sounds most likely a fuel pump. I would keep a test light handy, and the next time the thing does not start, make sure that you do indeed have power and ground present at the pump. You will only have power for a few seconds prime after a key on event, or while cranking. Here is a wiring diagram for you.
You can also have someone attempt to crank the truck while you bang on the tank with a nice heavy mallet, and see if it fires up. I do this all the time, just to get cars into the shop instead of towing them. I bet almost half of them with dead fuel pumps will start pumping with a few good whacks to the tank. I am guessing that when the brushes get old in the pump motors , that they get dead spots , just like a starter, and a good whack may be enough to make temporary contact. That will also help narrow down your diagnosis if the pump comes on with a few good smacks. I have done gazillions of Ford gas pu truck fuel pumps of that vintage.
Hope that is of some help.