Glad you solved your problem. I read about those screens but how can the rust make it through the first fuel filter, through the pump, then get stuck right before the carb? Maybe I am misunderstanding something.
Do you have the FSM in hard copy or online? It helped me a lot to do a baseline balance of the carbs and there are some thorough diagrams of the SUs.
My plugs were just as wet and black as you described since I bought it. I bought the Z Car "240z Tune-Up" dvd and did a half-way "tear down" of the carbs then balanced them. It took about 2-3 hours. After I balanced them with the meter, somehow (not sure if it was while driving it 50 miles in a week or during the tuning?) my front three champion plugs were completely dead and had me running on three cylinders. After putting NGK plugs in and gapping them, it runs so much better.