Hi Topcat...
Best bet here is to cut the cast iron pipe in two places using a sawzall and bimetal cutting blades and then use two
shielded clamps (see image) to connect pipe in between. If, say, you cut 12.5" out of the cast iron you will cut the PVC pipe about 12" long and install that in between the clamps...the two clamps each have RIBS that will fill in the 1/2" void. Here, you would slide the clamps onto the pipes and then FLIP the rubber sleeves back on themselves and then drop the pipe in between the sleeves and FLIP the sleeves back onto the pipe. Slide the clamps and all set, etc.
If replacing cast iron with PVC then you will need
cast iron to PVC shielded clamps.
Do NOT use the all rubber clamps with 2 SS bands...these are really just for underground applications.
Finally, when cutting cast iron pipe be sure to support the pipe and/or vertical load before cutting into the pipes. I cannot emphasize that enough!! Let us know if you need info. on this...
Let us know if you have more questions...
MARK