I built a 383 and am having problems with it. It starts to miss at about 5000 rpm. My a/f gauge starts to lean out and if i stay in it it will backfire in the exaust.Fuel pressure is rock solid when this happens.It only does it when I put my foot in it in first and second gear. In third gear it goes right up tp 6000 rpm. I have changed my timing , carb, intake manifold, fuel press, raised my float levels, changed plugs and tried different gaps I am at a loss. I know that I am not the only ever to have this problem someone has the answere. HELP ME OUT PLEASE