I'm rehabbing an 1888 Victorian that had freeze damage. I've had to acquire radiators from craigslist, salvage places, and repair a couple as well. I've run all the piping. My experience has been those "teeth" will snap using a spud wrench more often than not. I've had to remove the fitting the union is threaded in.
To remove the fitting, we (my wife and I) lay the radiators down and use a very large pipe wrench and an 8 foot long piece of old radiator supply pipe. The 8 foot pipe goes over the pipe wrench handle and we use all that leverage to turn the fitting and unscrew it. One of the salvage yards hammers on the fitting and that works. But they also end up cracking some of the radiators. I've found it's easiest to remove that fitting, get a new one, and coat it with blue pipe dope. I've never had any leaks. I've cleaned up and installed seven radiators so far and haven't found an easier way.