I would recommend doing some of the "common sense" checks first. Record mileage 2-3 times and take the average mpg. Look at a few of your plugs and check color to see if you're running rich. Check air pressure, PCV valve and air cleaner. Double check your timing and check your fuel pump for leaks. Verify your carb air/fuel mixture with a vacuum gage and make sure you're getting good engine vacuum at idle (15mg+) 20 is better. Once you verify the basics and if all checks out then that's probably all you're getting in mileage. If it was me I would also switch out the Holly for a well tuned Quadrajet, you'd have 750 as well and I'll bet you'll get better mpgs.