Do yourself a really big favor and buy a spacer with two outlets that goes between your intake and your water neck. It will give you two additional ports for sending units. And then buy one of these:
http://www.centechwire.com/catalog/a...ies/fc2p.shtml
I have this set up on my street rod and it works great, and you can dial in a temperature for the fan to come on. The unit is not bigger than a pack of cigarettes and very easy to install. The units that push into the radiator fins are not that accurate, and you will not get accurate temperature reading if you put a sensor in the petcock as that is reading cooled fluid.