Two ways of controlling fan speed:
If the fan, fans have a low and high speed, meaning three wire plug per fan then just a toggle switch can be used to run a fan at low or high speed.
If the fan, fans, only have two wires per fan then a toggle switch won't run them on anything but a high speed. You need a fan speed controller to have more than a high speed.
Hoping you have read through this age old post of fans here:http://www.montecarloss.com/community/ubbthreads.php?ubb=showflat&Number=183709#Post183709
I do believe the control of the electric fan needs to be done by something other than the human fingers. Using a manual switch after looking at the temp gauge doesn't cut it, usually to late.
So far the AutoCool unit installed on the Spal Dual fans has worked seamlessly. This info is at the end of the above post.
Yesterday 90 degrees, took the car out for a "quick" run, using an infrared thermometer saw 178 at the inlet hose connection to the rad idling back in the shop. The stat is a 180. No reason to think about the fan system again, it's automatic just like the factory fans run by the PCM.