I've been able to find lots of websites and forums that show how to connect cast iron to pvc in a straight section of pipe, but my cast iron pipes are rusting out right at a pipe union, where a straight pipe connecting into a large pipe in a Y fitting. The fitting Y section seems OK, but the straight pipe is crumbling. Is it possible to fit PVC into the iron Y piece, or do I need to replace the Y piece too and then connect that new PVC Y piece to the remaining cast iron lines some how?

It's easiest and cheapest to replace all the cast iron back to where it first has crumbled, using a Fernco rubber/hose clamp to connect a straight section of PVC to a straight section of iron.

Cut out the section of pipe you want to replace but leave about 16 to 18" of pipe sticking out of the cast iron hub. Drill out the lead between the old cast iron pie and the hub. Use a drill bit a little smaller than the gap between the the pipe and the hub. Drill lots of holes all the way around the pipe. Lead drills easy and fast. Move or wiggle the pipe sticking out of the hub around. Pull up, down, back and forth. When the pipe is loose pull it out. Clean out hub. Get a gasket sized to fit in the hub and PVC pipe. Probably have to go to plumbing supply house fot that.
Get a 10" length of PVC pipe with a hub. With a hub means PVC pipe with a hub on one end to accept another piece of pipe.

Cut off the hub of the PVC with enough pipe to fit inside the cast iron hub. Sand and bevel the end to ease pushing inside gasket. Put gasket inside the cast iron hub. Grease both the inside of the gasket and the outside of the pipe. Push or press the PVC hub/pipe into the gasket. That won't be easy. Hitting won't help. Cut a length of pipe to replace what you cut out. Use glue to connect at the PVC hub and a shielded rubber clamp to connect to the old cast iron pipe.