six barrel

First, 340's aren't economy engines. Second 340 TA engines damn sure aren't economy engines. They have high compression, a large factory cam,and huge cfm's for a fuel system. Disconnect the front and back carbs and see where your mileage is.
i was going to chime in "what is the MPG on a 6 pack...?" I seem to think a 4 barrel would be some where in the neighborhood of 13 MPG :)