What's the manifold vacuum at cruise speed and light throttle? You might need lighter springs under the metering rod pistons, or maybe just a carburetor with a lower CFM rating. If the throttle plates and venturis are too big, the vacuum drops drastically every time the throttle is opened, and the metering rods go to the full-rich position. You'd be better off by far with a stock-calibration Quadrajet. It was designed for the 305, and other GM engines, and there should be plenty of correctly set up carbs available. The Edelbrock is a generic replacement unit, and unless you've got a flow bench, it's going to be difficult to get an exact match.
Jerry
