You should be getting 15 MPG with what you have. I don't care if you are hauling a half ton of manure in the bed or nothing at all. That is what these trucks get in near stock condition.
Pull your spark plugs, then figure out why they are black and sooty. Could be a leaky accelerator pump dumping in extra fuel into the intake manifold. Could be too much carburetor for your application. You did not specify how many Weber 32/36 carbs you are running.
Carl
