I have a cast iron soil stack with cast iron branch to wc.I need to replace the branch with 4\\\" 110mm as it is corroding and is only 3\\\" diameter ( problems flushing big waste away) Could you tell me if I can connect a plastic branch into soil stack and if so how and how do I remove the cast iron branch,someone told me to grind the cast iron out and fit the plastic in, then make it good with putty is this correct?