The requirements for racing are going to be a little different, but for the street I would make sure that whatever fan you run is capable of at least 3,000 CFM and ideally even more than that. Try to avoid stuff that doesn't come with its own shroud already integrated, and try to stay away from shrouds that just look like a baking sheet with holes cut in them. The CFM for the fan, if it's a standalone fan that was tested without a shroud, might not be the same as the actual CFM it's capable of when it's paired up with a shroud.
I run the '95-'00 Ford Contour fans on my Duster, they fit a 26" radiator well- I only had to remove one tab on the shroud as far as modifications to the fans go. They're a dual fan, dual speed set up capable of ~3,000 CFM on the low speed and 5,000 CFM on the high speed. I run them with a Dakota Digital controller, but like any electric fan I'm sure that if you just wanted to trigger them with a plain thermostatic or even a manual switch I'm sure you could. That might be fine for racing but for a street car I'd never just run a manual switch by itself. And personally, I think as far as reasons why people have issues going to electric fans the #1 reason is picking a fan that doesn't have enough CFM (even some of the really expensive fans don't move enough air) the #2 reason is using an inadequate switching system. Being able set and change the temperatures when your fans come on and off is a big part of getting them to work with YOUR car, and simple thermostatic switches aren't good at that. The biggest advantage to electric fans are their improvement in efficiency, and that comes mostly from being able to run them only when you need to. Being able to control when you run them more accurately makes the whole system better.
I think the Contour set up is great. The fans themselves are pretty cheap, their design was factory tested to hold up to whatever warranty Ford was giving at the time which is more testing than any aftermarket company does. They have their own shroud, move a ton of air, and are pretty easy to control. The '95-2000 Ford Contour set up is only about 3.5" deep at it's deepest point, which is to the back of the motors. The rest of the shroud is about 3". With my 3 core Champion 26" radiator and the Contour fans the back of the fan motor is about 6" from the radiator support. The nice thing about a dual fan set up the motors aren't centered so the deepest part is offset from the pulley. Something else to consider when looking at a particular fan arrangement.
The install I did on my Duster is here
My "new" '74 Duster- or why I need a project like a hole in the head
I'll also attach the PDF that
@goldduster318 made up when he did this install, it's what I used to wire my fans up. My Duster just has a 340 in it with a probably a bit more than 400 hp, but it's .060" over, runs 9.8:1 compression with iron heads and I have driven it in traffic in temps as high as 110°F. I've only had the high speed of the fans come on a couple of times, both were when I was driving around in town and sitting at stoplights in traffic when it was 105°F+ out.
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