That heater nipple is not on all cars. My Lumina didn't have one. Simply a tube with an O-ring press fit into the engine.
I did have a Corsica a number of years ago that had one of these nipples. I didn't acually replace the nipple, rather I did a hill-billy repair by taking out the old corroded one, running a tap in the threads to clean out the crumbled metal. I then used some plumbing connectors, braised on a cut off piece of the heater tubing, then connected the two tube ends with some hose and clamps. Worked super but I don't have the car anymore.