Ditto shadangos idea. Quick and dirty way to do it is use a npt threaded extension in the intake manifold that is long enough to get the probe into the coolant. Typically a brass extension works for this, then an inside/outside thread adapter to get the temp probe to seal to.
After that you'll have to look at other alternatives to make it look nice, but the hardware store parts will get your temp guage attached for peace of mind.