I am installing a new basement shower. Drain is 3" cast iron, cut off 2 1/2 inches below cement floor with about an inch of space around it to work outside the cast pipe if I need to. Drain for new shower is 2" pvc. I need 2 inches below the cement floor to install the pvc shower drain connection. Local plumber suggested that I install a 2 inch pvc coupling inside the cast pipe and secure it in place using PC 4 (a cold caulking compound) - sort of a rope with a type of cement in it. I've never used this product before. Any suggestions?