Vince, it can be a pain sorting what temp range will work best, as you have a heat source you can't change (assuming ignition timing and mixture is correct) i.e. the engine, but can vary thermostat, fan effectiveness and fan thermo switch range. In sorting both Mustang and Cobra I ended up trial-fitting a few of these switches until I got it right.
I can only speak for the small-block Windsor. According to Bill Carroll's
Ford Cobra Guide of 1964, SAI swapped the factory thermostats for a 160°F (71°C) unit when they installed the 289 engines. This seems very low, but it's what I have in my factory-correct 5-bolt 289 cooling system and it works well. Basically, I think the cooling system in a 289 is so marginal that the thermostat opens early and the running temp is controlled entirely by the roadspeed, waterpump and electric fan. Dan Case has given me invaluable help in water pump decisions and I have chosen to go with a stock, not high volume, water pump which does not cavitate at high rpm as it has fewer vanes on the impeller. At Dan's suggestion (and in line with COB6029, the Haynes Museum car) I have also retained the mechanical fan on the water pump.
My fan thermostat is set to turn on the fan at around 90°C and turn off again at around 75°C, which it does in general usage. I have now achieved the point at which I can leave the car running with the bonnet closed and the fan will cycle on and off over a 30 minute period (the neigbours love me). To get to this, I had to fit the most effective 'puller' Spal fan I could find in 14":
http://www.jegs.com/i/SPAL/063/30102042/10002/-1 - and this works very nicely, rated at 1864cfm.
You may need a different temp rating switch as your fan switch is in the top hose, which will register earlier than the bottom hose (where the 289 switch is located). Only trial and error will tell.
Roger
ps as intimated in shep's post above, effective ducting to ensure the air cannot get around the radiator anywhere is essential.