I'd be concerned with the lead pipe. Any movement, rocking or up and down, will break the wax ring seal. As far as anchoring into the old plank subfloor, you could use plastic or lead sleeves into the planks then #12 stainless steel screws into them. I use lead sleeves into concrete anytime I need to attach or anchor anything and they will hold forever. Plastic would probably work OK as well. But, I'd sure deal with the lead pipe situation. Better to spend some time and perhaps $$ now while you're at it than regret it later when you have to rip up the flooring and redo the job.