Let's see, the problem you describe is that your truck doesn't run well when put under load. You describe the symptoms as stumbling and bucking. This sounds like a fuel problem. Yet you've replaced the fuel pump twice and the carburetor once.
You state that you can rev the engine with the truck parked on a hill, so it's unlikely to be a float level problem because that depends on incline not demand, and when demand is high the bowl won't be as full anyway, which will force the feed needle open further to compensate. Besides, the same problem existed before you replaced the carb.
I think red58's suggestion merits investigation. You could remove the fuel pump and measure the distance from the cam to the base and then compare that to the specs or to the actual fuel pump.
You should also investigate MtneerMike's suggestion. If you have a coil with a builtin resistor AND a ballast resistor, that's a problem as well as the 6 volt vs 12 volt problem.
