360 overheating -please help

Sometimes, all can be well, and an outside factor bites you. Mine was a 383 Swinger I built and sold. I sold it and part of the deal was a cam swap with a used one he had. So I did the work, drove it around, all was fine. It didnt make it the 20 miles to the new owner's house because it was overheating. He ended up fishing a dead mouse out of the water outlet on the lower tank (yuk). Now I put tape over all the inlets and outlets when the radiators come out....lol...live and learn...