That's weird. My 400 runs cool at speed and only gets hot in traffic on hot days. I don't have any of the fillers or baffles. Could be a bad fan clutch if you have one. I have a flex fan with no clutch. Why would a car overheat when air is going really fast through the radiator at speed? Is your vac adv hooked up and working correctly? This increases timing during cruise and thus keeps the engine cooler. Just a thought.