As you realize, cast iron is damn heavy. I removed my CI vent stack once and even though I thought I had it well secured before I started cutting, once the support of the lower end was removed, I had a 15 foot length of 4" CI pipe crashing on to the basement floor! I'd replace the stack with ABS or PVC because of the difficulty of supporting the weight of CI as well as ease of working with ABS/PVC, but just exercise extreme caution. I'd consider starting at the top and working down taking relatively small bites at a time if this is possible. Have help (muscle).