If the carb has a diaphram in it held down by four screws on the top and that diaphram is old I have found that the diaphram becomes stiff and won't allow gas into the engine, changing the diaphram usually sorts the problem. Usually if you get the truck started this problem disappears because the diaphram becomes warm and flexible. I'm ref to the impco carb and I'm not sure which carb your truck has.

Regards Titus