My 400 ran hot until I had all the correct air baffles in place. Do you have the upper ones that only came on the 400. If your car was a 350 and upgraded to a 400 this could be a problem. also, there is a lower air baffle that is often removed and discarded on old birds. Until I added this to mine, it ran hot. This directs air into the radiator vs. under the car. The strange thing with my car was that without this, it ran especially hot on the interstate at speed. Now it runs 5 - 10 degrees cooler. Ames sell a fiberglass replacement as originals are nearly impossible to find. You may also want to advance the spark somewhat too. The original cars had a TVS (thermal vacuum switch) that was to advance the spark when temps got too hot.
