Clutch fan options on my 340?

This make sense

Power Loss and Fuel Consumption
The 1970 Pontiac engineering brochure put it all in a nutshell with the following statement: “Fans. They cool. They also eat up horsepower. Conventional fans slurp up 20-24 horses at 6000 fan rpm. Flex fans, about 5 or 6. And clutch fans, about 4 or 5 horses. Take your choice. Why the difference? The flex fan blades flatten out at high rpm. Less resistance. Less drain. The clutch fan is thermostatically controlled. Cold, it runs no faster than 1000 rpm. Hot, it engages and picks up to 1800 rpm.