I agree with P.M.I USA. A 800 cfm squarebore carburetor is not going to help fuel economy.
Other than the carburetor, your system seems setup such that you should be doing better than 8-10 mpg.
Once you determine whether you need the lower end rebuilt or the heads rebuilt you could try the following:
1. I'm not familar with the Edelbrocks, but most carbs have a way to delay opening of the secondary valves. I'd try to add more delay to the secondary opening (make them come in later).
2. General purpose carbs are not calibrated for your specific application. They are usually "rich" from the factory to be conservative. Step 1: Adjust your idle mixture until you read the highest manifold vacuum. Step 2: Drive the car at 2000 rpm. Record the vacuum reading at that engine speed. Lean the primary mixture in increments until highest vacuum reading is achieved or you feel a "surge." Sometimes the vacuum will drop, but as you continue leaner it will come back up. So don't be afraid to continuing leaning it out. Since you are only adjusting the cruise mixture, you shouldn't damage the engine going to lean (it will just surge).
3. Finally, make sure your timing is set properly. Have it all in by 2800 rpm (mechanical). Crane makes a great adjustable canister and spring kit. Try connecting to manifold vacuum instead of ported. Some engines run better this way (especially if they have moderate to aggresive camshafts and low static compression). When connected, your idle speed will increase. Adjust it back down to where it was. The benefit is by adjusting the throttle opening more closed to reduce the idle speed back to normal, you will be using less air (and thus less fuel). Almost everyone I work with that has a classic happens to have set their cars this way. I tried it connected to ported, and then manifold, and found manifold vacuum advance worked better in my case. Since you are adding advance the mixture will probably richen, thus you can redo setting the idle mixture again and lean it out even more.
