You hard mount to a spacer, no clutch involved. Another bonus is dropping a lot of weight off of the end of the water pump shaft. The extra load from the severe pitched blades would likely make a clutch slip so direct drive like the factory did for decades.
I've run them on my dirt cars for 3 seasons at about a 5500-5800 RPM redline. After I took the fan off of the race car I put it on my 1973 T/A street car and it was on that car (5-6 years).
I borrowed it for an enduro car, the front end got crunched and the fan hit the radiator and the blades broke. The amazing thing is the plastic blades did very little damage to the radiator, unlike a metal fan blade does. I was able to solder 2 tubes up on that radiator and kept using it. I can't think of any downside to substituting a composite fan for the heavy metal clutch fan.
For $30 and some spacers and bolts, it's hard not to justify at least trying one......
