I have a single speed 16 inch Hayden fan on my 347. It has a 3 row radiator fitted and an adjustable thermo switch with the sensor fitted in the top radiator hose. It cools well . I have the switch adjusted to a few degrees above the thermostat rating so its on and off , as it should, until you are out on the highway then stays off with the air flowing through the core at speed. This fan draws 23 amps on start and runs at 16amps approx. A two speed will need a large capacity alternator and a wiring diagram to suit.