The stock heater lines are the same untreated mild steel as the main water lines, so don't be surprised if the engine ends are also messed up. The sheet metal spigot(s) in the engine are usually shot, too. BTW, they're pressed in, not threaded. Put a slug in the end so they don't collapse and you can lever them out with Vice Grips.
Whether you clean the lines out or chop them off, you'll likely find the last few inches are corroded to transparency on both ends. These days I tend to go for permanent solutions rather than temporary, hence my stainless steel recommendation. That's why I made ss engine spigots on my lathe. Brass would also work.