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I believe FAL (Flex-a-Lite) makes a direct replacement kit for your truck. I know they make one for my 88', so I would think they make it for the 99' too.
If your truck has a fan clutch, it isn't putting that much of a load on the engine. When the engine is cool, you can actually hold the fan still when the engine is running and the fan clutch is disengaged...however I don't reccomend trying this personally....I just happen to know that you can do it.
If anyone happens to know what factory electric fans will fit a Ranger, let us know. Usually a factory fan will pull alot more air than the typical inexpensive aftermarket fan, and can be had from a junkyard for about the same price. Usually the aftermarket temperature and A/C controls will work with most factory fans as well.
I have read claims of 5-15hp depending on engine, but never seen a before and after dyno. I am sure there is some power reclaimed by removing the parisitic drag, but how much is up for debate. Even though the total gains would be smaller on a 4 banger, I think you would be more likely to feel it. I don't think you would feel the couple extra ponies in a truck with the 4.0L. The "ButtDyno" would have to be pretty sensitive.