Plastic filler plug in the V8 radiator allows a probe to be attached. The wire is much further away from the fan belt that it looks here, it could be under the hose and still clear it, but going over the top makes sure it can't drop down:
Incidentally the shiny cylinder is a bit cut off an old alloy vacuum cleaner nozzle and retains the O-ring. When I fitted a new O-ring being soft and pliable I found it was being squeezed out from between the plug and radiator flanges because there are no retainer grooves in either the radiator boss or the plug, unless I left it slacker than I would like. This tube is a perfect fit round the flanges to keep the O-ring in place.
The plug is hollow, and incidentally the hex is exactly the same size as a plug so I keep a box-spanner and tommy-bar in a rear side compartment as well as this spare plastic plug.
A long stainless screw can be fitted through the plug with a spade terminal on top. Originally I just used the screw, but the electrical contact with the coolant was quite small which caused occasional false 'low coolant' warnings. A stainless strip bent into a U-shape and secured with two more nuts gives a much larger surface area for contact. In normal use the coolant level is right to the top of the header.
The wire is run back to the bulkhead alongside the temp gauge capillary and through one of the grommets.