Hmmm. Dual quads and a heavy foot, combined with some idle time and a bit of around town cruizin' could well equate to 10 mpg... Try some just down the road driving, keeping your foot out of the secondaries, and see how it does...
What size of Edelbrock carbs do you have on there? You don't possibly have a choke hanging part ways closed??? Is the linkage on the carbs progressive, and could it possibly be adjusted incorrectly allowing the secondary carb to open up too soon??