Did you ever "tune" the carburetor (jets, metering rods, step-up spring, secondary air valve) for your specific 402, intake manifold, cam, and ignition combination or did you just bolt it on and go? You should be getting about double the gas mileage that you are getting now. It sounds to me like this needs to be done. Also, how much fuel pressure do you have? 5-6 psi is best, 6.5 psi is MAX, if you have more fuel pressure than this you found one of your problems and you need to add a fuel pressure regulator.