Answer: -Go to your local plumbing supply house, armed with a detail sketch of what you want to do. Include some dimensions of pipe O.D. There are numerous couplings and adapters available to connect "anything" to "anything". Hope you're not moving the toilet too far from its present location next to the stack as there may be restrictions on max distance depending on loacl code (if that matters). -I have been researching this for my own house, and I have heard of several several ways: If the cast-iron pipe ends in a bell, as mine does, there are rubber bushings that one local (quite large) building supply store carries that seal variously-sized pipes into that bell. Before they stocked those bushings, the personnel at that store advocated cutting the bell off the cast-iron pipe and using a no-hub coupling, which clamps around the outside of both pipes. The "Basic Plumbing" book from Sunset describes sealing the plastic pipe into the bell with oakum and lead wool. My brother-in-law tells me that book from Popular Mechanics(?) describes a plastic spigot fitting which is shaped just like the spigot end of a cast-iron pipe and thus has an external flange that seemingly would improve the mechanical stability of the joint and make it easier to pack the oakum. At our local Home Depot, they stock a ABS spigot, but the personnel at that store do not know anything about them, the store only stocks two sizes, and the store does not stock the oakum or lead wool. I happened to catch one of the TV home-improvement shows (Vila?) where the guest plumber sealed a piece of PVC into a cast-iron bell with oakum and _molten_ lead!! He said that the plastic would survive if it wasn't stressed until the lead had cooled.