I have a 62 impala SS with a 327. It has the 300 hp heads (double camel hump) and intake manifold. I have replaced the water pump with a high flow unit, and put a high flow 180 t-stat in it, installed a new thermo fan clutch, and had the raditiator boiled and cored. I cannot get this to run under 220 on an 80 degree day. On warmer days, or with the factory AC running it really gets hot. The block's cooling jackets look very clean when veiwed from the water pump and t-stat removal. The only thing I can think of that does't seem right is that the exhaust manifolds may be off a 283. The diameter going into the exhaust pipe is 1 and 7/8 inch and I'm thinking that's to small for those heads. Could that cause overheating like this? It runs hotter at higher speeds 55-75 than slower speeds 30 to 50.