We are preparing to tie into an existing cast iron 2 in stack with a new laundry drain. there used to be a laundry sink draining into it via an ABS trap but this is poorly situated for what we are doing.the question is this. My understanding is the best way to tie into the cast iron is to cut it and attach an ABS fitting with sleeves and clamps. But there is a place where a cast iron fitting is screwed in. My husband think the best solution is to remove that fitting and screw the abs fitting in its place. So what is the best solution?