Scotty, the pulley ratio is more important than the size, should be 1:1 (same size) or 1:something less (fan pulley is smaller than the crank pulley) Be sure to look at the correct groove too, sometimes Ford have multiple ODs on the crank pulley, compare the one that drives the fanMe and my friend Larry are going to try a smaller water pump pulley I measured my water pump pulley and it measured 7" so that's means the fans spins at low rpms , with a smaller pulley the fan will spin much faster keeping the temperature down im going with a 5.400 dia water pump pulley, and we are going to check the timing and compression, and vacuum, but we have to get my Cadillac engine back in my caddy first, I will let you know in about a week
Ethan - If your cooling is better once you put the stock fan on, chalk it up to experience and realize that it takes a heck of an electric fan to do what a stock fan does. Like it or not to the hot rodders, the benefit of electric fans is power (during the drag race) and the ability to cool between rounds with the engine off, they are not more capable at low rpm or in traffic unless you buy monsters and have the room for them, and in those cases, they draw more electrical load. In the end, it's pretty hard to beat a good fixed fan.
One other comment, lets say you gain 7 HP with an electric fan, but the engine is 15 degrees hotter as it creeps up, although you would lose HP with a fixed fan, the lower temp allows more stable tuning and a more consistent performance, so it's all in the margins, you won't even notice. Last thing, 400 dollars in electric fans can make up that power if spent somewhere else too...