Hmmm, I stand corrected. With that relatively large motor (the 6 banger), little ambient air flow and hot summer temps, your idea for the big Taurus fan sounds real nice.
A J/Y alt if needed sounds like a good plan.
The OEM two speed fans are generally controlled via the computer. For instance, on a 94-95 GT, there's a high and low speed relay and the EDF relay control module (solid state). Simply however, it's the puter that controls things.
I don't know what you have planned for controlling the fan, but a toggle switch and relay for each speed would likely work pretty well. You'd want a toggle for each speed since having both engaged at once really stresses the motor (and makes the fan spin only a hair faster than low speed anyhow). Slantman's site has a diagram for how to wire such a beast up (with LED's to let ya know what speed is on).
Good luck bud.