A SPAL fan is a major upgrade over the stock S fan.
The fan is wired easy enough through a single pole or two wire thermo switch. The main switched power for the bike is an orange wire (check it) that runs to most of the bike and can be tapped into somewhere near the head tube. This connects to the positive side of the fan.
The negative side of the fan connects to the centre pole of a single wire thermo switch and the earth return is through the frame. Ensure that there is a good ground path. The second wire on a twin wire thermo switch is a ground return through the harness (B/W). I'd suggest the latter as this is exactly how the S and Oz E is wired.
The fan has a plug and the supplier of the fan should be able to provide its partner built with a couple of feet of wire. Same with a thermo switch plug.
Wiring a fan properly is dead easy on the DR-Z if you can get a clear picture in your mind of what is required.
The standard S thermo switch is rated for 205F. You might want something a little cooler - say 195F. Check your LH radiator for the fixture for the switch - some seem to have it while others don't. If not then a hose adapter is probably the best solution.
There are no dramas with clearance under any tank with the fan on the lower portion of the LH radiator.