Not the way to do this...trying to make a compression fitting seal to the lead closet bend isn't going to work well. It may not leak until you have a plug, then it would leak out because of that poor seal. The lead bend usually is attached to a brass sleeve that is held in place in the cast iron hub via a leaded fitting (made with oakum and poured lead). That can be removed by drilling out the lead and prying the pipe out, then cleaning up the lead and oakum. You can buy a rubber donut to fit in the hub and it will seal to a piece of pvc or abs drain pipe and you can rebuild from there.