Sounds like a coil or condenser breaking down under load. In my experience, when a condensor breaks down, it will cause erractic spark timing, which leads to the backfire in carb. A bad coil is easy to spot in some cases. Drive the truck till it dies and go outside and touch the coil casing. (be careful
) If it is hot to the touch, and I mean hotter than the engine, then it is most likely shorted.
Personally, I'd change the condensor first,it's easy and cheap. Sure doesnt sound like fuel problem
