Will the old cast piping marry up to pvc?

Hi BV120...

Yes, Cast iron and PVC can "marry" each other as long as you purchase a cast iron x pvc SHIELDED CLAMP (picture on left). Do not use the all rubber coupling with two SS clamps (pic. On right). These are not intended for your project.

Common names for shielded clamps include proflex clamps or mission clamps.

They are usually available at home depot or Lowe's type stores. If not available at these stores in your area then they can definitely be found at a local plumbing supply company.