I'm running an 18", 7 blade flex fan from a '69 Caddy on my '67 GTO which is stock. Pulls a ton of air, is quiet, and runs cool. Running a 19.5" clutch fan with a Hayden clutch on my '65 GTO and it runs cool and works fine. Whatever you run, use a 7 blade fan that is at least 18" in diameter to pull enough air through the core.
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Jeff 
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