I'm roughing in my basement bathroom... I've dug the trench and uncovered the old pipe which is looking pretty rough (my house was built in 1945). My question is, how can I ensure after I cut the pipe and piece in my PVC that I will get a good leak-free connection with the rubber couplers? Is there anyway to clean up the old cast iron?

Use a Rasp. Or heavy sand cloth

"how can I ensure after I cut the pipe and piece in my PVC that I will get a good leak-free connection with the rubber couplers? Is there anyway to clean up the old cast iron?"

I use a wire brush to clean off the dirt and crud. I advse using No-Hub Bands to join the two instead of rubber couplings. Fernc Couplings are OK if you can strap and secure the pipes but a rubber coupling under the slab can sag and produce a bell in the joint. No-Hub Bands give the joint a more rigid connection. Good luck, Tom

I'm roughing in my basement bathroom...I've dug the trench and uncovered the old pipe which is looking pretty rough (my house was built in 1945). My question is, how can I ensure after I cut the pipe and piece in my PVC that I will get a good leak-free connection with the rubber couplers? Is there anyway to clean up the old cast iron?

All of the other suggestions are great ones, but make sure there is no raised lettering or a casting seam at your tie-in point -- If there is, you can knock them down with an angle grinder or a file.