I am going to suggest vacuum is a better way to go. With vacuum secondaries, the engine opens the secondaries when the engine need the extra air. If you have a double pumper, the engine may not need the air, it is at too low RPM. Then air velocity falls too slow through both the primaries and the secondaries, and the carburetor loses its ability to accurately control the fuel air ratio.
The idea with a double pumper carb is the secondary accelerator pump squirts a shot of gas to cover up the opening throttle bog, and by the time that gas is burned, the engine has increased RPM to be able to use the air coming through all those wide open holes, and then can control the mixture.
With a heavy truck, towing something, the rpm will not have had enough time to increase pulling the load, and the engine may just briefly run off the accel pump shot, and then bog down, being too lean.