How to work with existing cast iron?
Retrofit work differs from new installations in two ways. First you don't usually have as much room to work. Second, in addition to extending the new, you'll need to adapt to the old. In the case of cast iron, that means finding the best place to splice into the line. In some cases, it means making the whole project fit current code requirements. Once you cut into grandpa's work, you lose the benefit of the grandfather clause.
Where you choose to cut into an older cast-iron piping system will depend on what you hope to add or improve. If you've built a home addition that includes plumbing, you'll likely need to cut into the stack in the basement. If you're adding a bath in a basement, in an area without rough-in piping for a bath, you'll need to break the basement floor and cut into the underslab soil pipe. If you are simply remodeling an upstairs bath, you may not need to cut into the stack at all. You may be able to simply cut into the existing horizontal toilet piping, just beyond the stack fitting. You can then extend all the fixtures from this full-size line. You may need to cut the stack in the attic to tie in a new vent, but that's usually easier than breaking the stack in a wall.
Hazards of cutting vertical stacks. If you need to cut out a section of cast-iron stack on a lower floor, it's important that you make sure the stack above the cut can't come crashing down. Most stacks won't fall when cut, because upper-story vents or branch lines hold them in place. Still, it pays to check, especially when you consider the weight of a 20-to-30-foot pipe. The most surefire precautionary measure is to install a stack clamp that grips the stack and is supported by nearby wall studs.
If you can't locate a stack clamp or the local wholesaler won't sell one to you, you'll have to improvise. If necessary, you can make a clamp with strap iron and bolts. In most cases, you only need to support the stack until you can splice in the new fitting. Then you can remove the supports. Dimensional lumber works well as blocking. If you're working in the basement, for example, and you plan to cut out a section of stack just below the toilet fitting, jam a 2*4 stub between the toilet T-fitting and the basements, a short 2*4 jammed between the floor's soleplate and the sink's branch arm will work. You could also go into the attic and prop lumber under a re-vent just as it enters the stack. In this case, you'd use the lumber to bridge two ceiling joists or to block up from the plumbing wall's top plate. A final option is to suspend the pipe with hole strap or lightweight chain. This is the best way to support horizontal piping.