the stock CST is located to the right of the distributor in the lower intake manifold. It is made of brass and has a small wire connected to it.
you do not want to use a T fitting as you will get inaccurate readings.
Drain about a gallon of coolant from the system
remove the old CST
put a small amount of pipe dope or teflon tape on the autometer sending unit fitting and install and then the autometer sending unit it in the hole where the stock CST was.
Don't bother worrying about the stock temp gauge as they are inaccurate and that is why you are installing the aftermarket unit
If you are dead set on having both the stock and autometer gauges, leave the stock one where it is and get a thermostat housing with a fitting for the autometer sending unit. You will need to replace the factory thermostat housing
make sure you have a good engine or chassis ground on the gauge or you will get an inaccurate reading