Hi. I am installing a downstairs toilet. My existing stack is cast iron. I don't really want to replace the whole stack so I'm hoping to to replace a small section at the bottom with a plastic t section. My question is if I chop the section out and replace with the plastic t, would it still support he weight of the full cast pipe above? I'm planning to chop the pipe at ground level and use one of these coupling sockets for the plastic to fit into 110mm Soil Pipe Slip Coupling Double Socket Any advise would be appreciated before I go ahead.