Looking at pictures of the 216, the temp sender is at the back on the drivers side of the head. There is an adaptor that screws in to the head that holds the capillary tube bulb. Remove the bulb nut and original pieces, then remove the adaptor. Your electric gauge should come with the right fittings to screw into the head. The electric sender needs a good ground, so go easy on the teflon tape.
Drain the coolant down below the head hight, no need to drain it a out.
To test your OEM gauge, pull the bulb out of the engine and place in a cool water to get any heat out of it. Then take a heat gun or hair dryer on low heat and slowly heat it up. If your gauge tracks eratic, go the electric route, but if it acts normal, check thermostat, radiator, and/or water pump for any issues.
