You only need to change the cap if you have the right tank and hose setup and want to suck the overflow back into the rad when it cools down. In that case the cap has a double gasket on it vs a single gasket.
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1966 Strato Chief 2 door, 427 4 speed, 45,000 original miles
1966 Grande Parisienne, 396 1 of 23 factory air cars (now converted to a "factory" 4 speed)