I have an up and down condo where I own the second floor and do not have access to the first floor to make changes.. The master baths are stacked and use a common 3" vent stack. I want to make my master bath slightly larger and move the WC back toward the vent stack about 12". This puts it right on top of the shared stack. Can I mount my WC directly on the 3" stack assuming I continue the vent by coming off the existing 3" stack under the WC?