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A standard duty clutch fan was what was equipped from the factory, which would be a Hayden 2705. This was even for AC cars.
Most owners upgrade to the Heavy Duty 2747, which is JUST barely applicable for the 18.5-19" Pontiac 400 or Olds 403 fans. This is what I use and it cools very well.
Still, others overkill and use the Severe Duty 2797. The issue with using this clutch is that it is meant for fans with a much larger pitch than any factory fan, and as a result it is pretty much engaged 100% of the time. This overcools the motor as well as robs engine power due to how stiff it is.