Well if you don't like the way it hauls heavy loads now with a small torque cam, you definitely won't like it with a mild performance cam. Bigger cams shift the torque curve upwards in the rpm band. The best way to turn that motor into a heavy load hauler would be to stroke it to a 383 and go for all the compression you can with the fuel you use. If it's just a limited use truck shoot for 9.5:1 - 10:1 and use premium fuel.
Also, if you have 882 or similar heads, they are junk, even on stock engines. Consider upgrading to L31 vortec heads, or if you want to keep your intake look for some good used World SR Torquers (they are just too expensive new) or Dart Iron Eagle S/S heads. The superior swirl characteristics and port velocity of any of these heads will improve power and efficiency.
A smaller vacuum secondary carb will definitely be better than a mechanical secondary carb for hauling loads. So I'd skip on the 670 double pumper, for this application. And you already have a good intake for a street truck, so no need to swap that out.
Before doing anything, are you sure your ignition timing is optimized. Then, is carb calibrated properly? Maybe your gearing isn't appropriate for the loads your hauling.