Ridge,
The common answer is that the accelerator pump is not working properly. With the air filter off, you should be able to see a squirt of fuel in the throat of the carb when you goose the throttle. That squirt gives the engine a little oomph when the vacuum drops suddenly when you accelerate. I would say it's not uncommon for the pump to not work after a rebuild - carbs are fairly complicated critters, what with all the things they're supposed to do under different circumstances. Check your rebuild if you don't have a squirt.
Another possible issue would be timing. If you're timing is off too far, the truck will be sluggish. You can loosen the distributor hold-down bolt while the truck is running, and "tweak" it one way or the other to see if it sounds better. Then tighten it up again, and give it a test-drive. Don't make any major adjustments, unless you are familiar with resetting the base timing. (top dead center, timing mark, timing light, etc.)
Other possibilities are plugged exhaust, inadequate fuel delivery, and towing an overly-large anchor behind your truck . . . ;-)
Check that squirt first and report back.
Hope this helps,
-Michael
