Return leg radiator

Hello All,
I'm getting in some last minute tweaks to my one pipe steam system before the cold. One is that I want to hook up a radiator in my garage on the return leg. This was the original configuration of my 1926 home, and a giant radiator (with a big hole) still hangs on the wall. In any case, I have a spare large radiator, but how the heck do I remove a second opening to make it a two pipe radiator? The plug hasn't budged with banging, burning and general swearing at it. Any help would be appreciated