Alternate placement for a temp gauge sender: This will allow you to keep the stock gauge for looks. Use the heater feed that comes off the intake manifold. Cut the rubber hose that connects the manifold water feed to the heater and splice in a tee adapter for the temp gauge sender. Be sure to use the same water feed line as the ECT sensor. That way you will get the most accurate temp readings.
Tee adapter info:
Make a pilgrimage to your local hardware or home supply center and get some copper pipe and a tee that fits the temp gauge sender. Solder two pieces of copper pipe onto a copper pipe tee with threads in the tee part. Find the correct brass fitting to match the temp sender threads to the tee fitting.
Keep in mind that the capillary tube on mechanical temp gauges cannot be cut, shorted severely kinked or damaged in any way. It is one piece from the gauge to the temp bulb that goes in the engine coolant.
You main challenge will be getting the wiring or capillary tube through the firewall. Do not use the steering column exit path for any wiring or gauge plumbing. Make sure you seal any hole you drill in the firewall with high temp red silicone sealer.
The boss cast into the thermostat housing places the temp switch on the radiator side of the thermostat.
If you put the switch in the thermostat housing, it only shows the correct temperature once the thermostat opens up.
The best place is in the same coolant flow path as the ECT. The computer uses the ECT to tell it what then engine's coolant temperature is.
The drawing I posted shows how to put the thermostat switch in the ECT coolant path.
