If not saving the pipe and there is a bit of room to swing, I agree a sledge will do it quickly.
Removed all my all CI pipe from the very shallow crawlspace laying on my side with a 3lb straight pein. Few good whacks on the bell joint and it will crack right open. Wear a respirator, lots of dry rusty **** will come out.
Alternately, if trying to save some of it, sawzall from the outside or inside as you see fit. Respirator again, for dry rusty **** and lead. CI cuts easily with a good fine tooth blade. I would love to try one of those new carbide tipped ones on it, but alas it is pretty much all gone in my place now!
I am a great fan of PVC for sewer lines and replace old clay, iron or cast iron pipe whenever the need arises. It just isn't worth saving generally.
I suggest skipping the heat. Even a regular lead joint will take a ton of heat to get apart (fire hazard) and the Oakum will probably burn with a nasty stink. It is a different animal pouring molten lead into a joint than it is making molten lead inside an existing joint.