Cusser
+1y
I'm thinkin' that the first thing I'd check would be the electrical part of the ignition switch. Symptoms are the same as I had with my '88 B2200 15 years ago. Your Weber carb is not a factor in this (I have one too).
The circuits for the starter are different than the "run" circuit. The electrical parts of these switches typically go bad like this at about 20 years or so. Next time this happens, do not switch off the ignition, just pull over, open the hood, and see if you have 12 volt positive to the coil positive terminal. If not, it's the switch. You could temporarily jump to the parking light circuit to the coil positive, then turn on parking lights to run the truck and then replace the switch.