If you can examine the area around the crack, you might probe to see if it goes all the way through to the inside. That might be the only problem area if it causes a weak spot while you are working your way down. Most stacks have so many course of brick and refractory tile that unless the crack is like a huge gap, it probably won't cause a problem. As long as you have room around the base to position your lift, that 120 footer will do you fine. Work your brick to the inside but coordinate the cleanout as you go so that you don't over load the walls and that you don;t have clean out going on while demo is going on. I have heard too many stories of near misses with falling debris. Be safe!