A rusted pipe can be heated, beaten and cursed. To loosen a rusted pipe needs a variety of methods. PB Blaster works well, but you may eventually have to cut it.
We have and use the usual lubricants in the hardware store. Liquid Wrench and WD-40 are the most common. A product we started using about a year ago is PB Blaster. It is our current favorite choice when looking for help when removing a broken or frozen bolt. Spray it on the joint and wait a minute or two and your pipe should break free.
An alternative to PB Blaster is to use a plumber’s candle to get the pipefitting broken loose. The candle is not to heat the elbow, but to drip wax where the two meet. Heat the elbow with a propane torch until it is hot and then feed some wax, paraffin or candle wax, so it draws into the joint. Be careful not to drip hot wax on yourself or the floor. Now try the pipe wrench to loosen the pipe.
If it is still stubborn, use more heat, more wax, this time hitting the fitting a time or two. This will enable the wax to get in the pipe threads and lubricate the surfaces so they will come loose. If not, go to Plan C.
Using a hacksaw or sawzall, cut the pipe at the best position for the new radiator location. Then rent a hand pipe threader. Make sure it has a long handle. The longer the handle the easier it will be to crank the die to cut new threads on the steel pipe. Use plenty of cutting oil while you are doing this procedure. If the diameter of the pipe is over 1 1/4-inches you should brace for a little battle.
Assemble your new fittings and pipe, using a quality pipe dope rated for steam.