My friends Mustang 289/302 has it on the front of the manifold casting like most easy to access under the water neck as you mention... But my Mangusta has the two tube manifold like a 'Cobra' as I can see... thats why I can seem to find it...?
Oye! You or the previous owner are the only ones who will know if it has one, or where it is. There is no standard stock place where it would always be installed to.
There is an aftermarket self contained housing available and designed to be placed in line somewhere, wherever you choose to.
The problem with it is that it has no bypass in it to help in warmup. It's also possible that you do not have a thermostat and separate housing installed at all and the issue is elsewhere.