My house was built in '48 and the original shower was plumbed with a 1-1/2 drain. I've gutted this bath and installed a new neo corner kit and thought I would just connect a new 2" pvc drain to the original cast iron soil pipe....but note where this connects at point A. in the photo: I couldn't find a dounut adaptor to use with 2" pvc here. After looking at things closer...this "y" is a 1-1/2" and will not take a 2" line into it...nor do I want to reduce the drain downstream of the trap. So....do I go with the original size of 1-1/2". It worked fine for us fo rthe last 15 years...so it should continue to work for another 15 I guess. The only other solution that I see is to remove all of the old cast iron stuff...and redo it with new PVC. This pic is directly under the toilet with a vent at point B. This stack goes through the block wall and into the basement next to the hot water heater: Then into the slab and out to the sewer: Back in the top pic...at point A. This is where I need to connect both, the shower drain and the sink drain. I'm a skilled diy'er but lacking experience with this stuff...... thoughts on how to approach it? Sincere thanks, Joe.