ludwig;
At NAPA (for example), fans are rated in 'horsepower handled'. A sample: the 12" fan will cool 60 HP. I added one of FoMoCo's own 12" front-mounted fans from a wrecked Tempo to my 200 Fairmont (Cost me $10 from the boneyard). When stuck in heavy traffic, it's just the ticket.
I also changed to a clutch-type mechanical fan, replacing the original flex-fan. At low speeds, the flex fan does not affect HP, but at high engine speeds it can, because it never 'lets go' of the belt drag. The first thing I noticed after the change was that I could climb the very steep hill near my house at 7 MPH faster in 2nd gear, which is a BIG change. My gas mileage got better, too.
If you have a Mustang, the fan clutch trick may be difficult to pull off, because the Fairmonts have about 1" more clearance between the water pump and radiator. But, if you can do it, it's well worth the trouble.