What kind of condition is the tank on that truck in? If the tank is rotting on the inside, then pieces of it will get sucked into the filter in the carb, and cause fuel starvation at higher speeds, due to heavily restricted flow.
This can happen with any carb, not just a Q jet. One downside to the Q jet design, is that you can't inspect the main filter without removing the fuel feed line. However, you can add another filter in front of it to give you an idea of how dirty your fuel system is.