Why would a '66 Impala overheat in traffic?
Having owned several 66s sense 66 I can tell you that B bodies (Caprice Impala Belair and Biscayne and associated wagons and SS models) that had A/C or a big block came with clutched fans and fan shrouds from the factory (6s and low power small block v8 without AC usually did not). A fairly simple test and adjustment of the fan clutch was printed in an article 1990 article in Corvette World " Cool Down by Henry Mollatch" to paraphrase Start the engine cold Run about 2 min shut off the engine from idle speed while watching the fan it should make about three revolutions after the motor stops. Then warm up to operating temp and repeat the test the fan blades should stop much faster. Note 2 that the OEM fans for pre 67s were set to about 160 deg F and the currently available stock is usually set to 180 or higher see article for adjustment procedure. My current cruiser is a Lemonwood Yellow 66 Impala SS 396 Convertible with AC.