I had the same issue with my 79 4WD for a couple weeks. I was ready to sell the thing and buy a newer truck I was so mad at it haha. I'm glad I didn't. The thing that was seemingly wrong with it was the two relays under the hood. They're behind a cover that is kind of a bronze-ish color. Take that cover off and tap on those two relays kind of like you would the starter (with the wooden handle of a hammer) very gently.
If that doesn't work another thing that is kind of sketchy but works is get a really thick wire, like a 10 gauge, and touch the positive of the battery to the negative of the ignition coil. That worked too after another couple weeks of messing with all of the electronics.
Good Luck!
--Nathan
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Hannah - '79 4WD, 32/36 Weber, and a Pacesetter header.