Edelbrock Air Gap on Magnum 318 Problems

I do not think that intake manifold vacuum is the right source for the distributor advance pot, that alone might be causing the stumble. If you check out page 6 here, you'll see Holley indicates that the port at 2 o'clock from the choke is the correct source for advance vacuum.

If that's not the problem -- I am not a carb expert, but everything I have read about off-idle bogging basically points to accelerator pump being the wrong size, either its too big and it's making the motor run rich until you stop moving the pedal down, or it's too small and making the motor go lean until the power valves come in from the increased airflow. You can either fiddle around with those in a hunt-and-peck way or you could beg/borrow/steal an oxygen sensor (bungs are pretty cheap and I _hear_ you can just jbgoober em in, but my muffler shop welded one in for me for $15) to help you figure out which direction it's going.