IMHO, the "marginal" cooling issues with early Mustangs is more often than not caused by some deficiency in the cooling system or tuning, such as.... a. Poor quality radiator (too little tube area or insufficient fins per inch density). b. Plugged radiator tubes. c. Radiator painted with enamel paint. d. Poor quality water pump (poor impeller design, supply holes to block not indexed well to front cover). e. Lower radiator hose closing due to pump suction at higher rpm (lack of spring). f. Insufficient spark advance. g. Too much valve lifter pre-load. h. Restricted exhaust system. i. Rust and scale in block (flush with "Shout" laundry spot remover followed by "Mercedes Douche"). j. When it's a Cleveland, not using the C-specific thermostat and block-off plate. k. V-belt slippage. l. Mismatch of steam holes between block and heads ("early" block and "late" heads or vice-verse).