The specific clamp that Jennifer is referring to is called a riser clamp. It can be purchased at any plumbing wholesaler. It is what i use to support the pipe, because it is a guarantee of no movement. Once the riser clamp is attached to the pipe, you screw 2-3/8 hex head lag screws up into the wood. This will support the pipe ridgidly when you cut it. It won't move at all. In addition, you can cut the cast Iron with a band saw, in addition to a snap cutters, but a snap cutter is the best solution.
But before you do all that, you need to consider the fact that you will need to revent the waste from the toilet after you tie it into the stack. Every fixture that is tied into the plumbing stack (that is legally installed) has a vent that mirrors the waste line and runs up to the roof to relieve the sewer gasses that are in the sewer line. With out a vent, the sewer gasses end up coming out the toilet itself. They smell pretty bad. I would get a book by Black and Decker on Home Plumbing Remodeling. About 9 bucks, cheep at any cost.