OEM fans work well, they are balanced and they last a very long time. I tried to get Ray Clark to put an electric fan on his 'bird at one time. He went to his local bone yard, found a Crown Vic and had them remove the alternator and fan. For some reason, Ray decided not to install them on his car so I bought both pieces from him and I mounted both on our '59 Galaxie Y-block.
The fan I'm using is one big fan in a molded shroud (like most of them are). The dual fans are ok as well... Ford turned one fan on all the time with the A/C compressor, and the other as a backup.
When you get the alternator with the fan you KNOW they are matched as the alternator is certainly large enough for the fan, even if the car sits like a taxi cab.
When I fitted the fan to my application I noticed there was a lot more room in front of the radiator so I spaced my radiator back toward the engine 1-1/2" and mounted the fan in front. This worked out great because the wires only needed to be reversed. These fan motors do not use a chassis ground because they are mounted in plastic. Another advantage is, the fan is not on the hot side of the radiator. It pushes colder air into the radiator and the fan motor never gets hot. Win, win. Works great.
I am using a temperature sensor mounted about half way down my radiator, just in case the coolant level dips. When the radiator gets hot the fan turns on. Just that simple. Just because the engine is up to temp does not mean the fan needs to run because wind removes heat from the radiator as the thermostat regulates the engine temp.
In Michigan winters, my cooling fan may not turn on for months. This is a tremendous gas savings and one of the reasons why modern cars get away with using smaller engines. Another feature new cars use is the WOT switch (wide open throttle). If you floor the gas pedal, this switch shuts off your A/C, alternator, cooling fan, etc., to deliver maximum HP to the wheels until you lift your foot after you get off the tracks. - Dave