You can get a thermostat housing with a threaded port for an extra temp sensor like this one, https://www.summitracing.com/parts/rnb-902-1001/overview/year/1965/make/ford/model/mustang

I might consider putting the fan's temp sensor in there and leaving the original one in the intake since it's already there and wired in.

 

That is exactly what I did, shown here and discussed in another recent thread

 

That thermostat is exactly what I have on my car, and exactly where I am about to put the fan sensor. I did some asking around and I plan to leave the engine temp sensor where it is (in the front of the manifold on the drivers side) and run the fan wiring down the passenger side and mount the sensor in the thermostat housing threaded hole.

 

Great! I was searching for the wrong keywords. Thanks for the solid answers!

 

Any empty port on the back of that intake that is plugged?

 

You can also get heater hose adapters that the temp sensor screws into. I used one when I wanted an additional temp gauge in my 71 but didnt have any ports to use. Just use the hose that is going TO the heater core as opposed to the one returning from it.

 

Simple. You get out your electric drill and a nice sharp 9/16" bit, drill a hole into the intake manifold into the water passage near your OE temp sender, search for... oh, there it is!....your 3/8NPT tap, and spin some nice new threads in the hole. Install the new sender, plug it in and VOILA!