You may want to consider the SPAL PWM controller.
http://www.spal-usa.com/
I have hooked it up to twin puller fans on the GT40 and it is great. You can program when the fans go on. 1 fan goes on at 175 degrees at half speed, then goes on full speed at 180, then both fans go on at 190. When the temps drop down, the reverse happens. I used the A/C signal input wire to hook up an override switch. It wasn't too difficult to wire up. I just had to read the instructions 4X.
