Snap-cutters are the professional tool to break cast iron pipe, but they take some getting used to and you have to build up some skill with them to avoid shattering the end of the pipe so that you will end up with a fairly even cut. Since you are only wanting to remove 1.5" from the end of a pipe, you will find that the snap-cutter will end up shattering the pipe. So I would suggest for only small jobs like this where you are only trimming off an inch or two, I would just use a carbide recip blade and get a nice easy clean cut.