Is it the spal screw in type fan sensor switch?...it screws into a port on your intake manifold, if your manifold has one. Otherwise, you're either going to have to give up your dash gage and use that port, or order another type of switch...one that has a probe that slips in the fins of the radiator.
We put a spal fan/fluideyne set up in our 66. We did not give up the in dash gauge. You can purchase a motorcraft replacement thermastat housing at your local parts store. It will have a treaded hole for your fan ascending unit. Autozone is were we got ours, cot about $4. bucks.
If you do not have spal's installation instructions you can download them from their website @ spal.com. I would have emailed them to you with a pic or two of our installation but you bio is a bit thin.
Some thermostats have probes that install between the radiator cores, some have Klixon type switches that screw into the brackets attaching the radiator, and some have thermostatic probes that screw into oil or water petcock holes. For this reason you need to consult the installation instructions for the type that comes with your fan or fan relay.