Great question — answering as the UphillTech team:
I'm not up boiler specialist or plumber, but I've worked with plenty of cast iron and galvanized pipe. I think your approach is reasonable. It sounds like the job needs to be done, so just go in knowing you may have a little more work to do than planned. Stabilizing the fitting to be left behind is key to avoiding damage upstream. Make sure you securely mount a pipe wrench to each side of the joint as you work, and balance forces carefully. If a pipe doesn't break loose with moderate effort, I would heat the outer component with a plumbing torch, causing its diameter to expand. I don't know what thread sealant would have been used in 1939, but this may also serve to soften it. Protect...
Full answer by isherwood: https://diy.stackexchange.com/a/332372 (CC BY-SA 4.0).
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