As 75k52wd said, the clutch lets the fan stop increasing rpm at some point(unloading). A flex fan flattens out at a certain rpm(unloading). A factory fan with no clutch has no way to unload and may stall the water pump if the fan is catching enough air.
Fan clutches are expensive. At least the good ones are. If you want to go with a direct fan, you need some sort of flex fan. I recommend the plastic one from Flex-a-Lite. The aluminum flex fans will hurt your truck or kill you when, not if, it breaks. The steel flex fans will last a bit longer, but are still dangerous. The plastic flex fan works great with way less danger of breaking.
Or dump the fan altogether and run an electric fan. You will have to upgrade your alt to at least 100 amps too. Do a search for electric fans. There are a lot of threads about this.
The simple thing would be to find the correct clutch length and install that clutch. But simple is not as fun.
