Since freewheels are cheap, if you want to save the wheel you are going to need a heat source (hot air blower or torch) some good penetrating oil like PB Blaster and a pin spanner. Get the freewheel warm, then squirt oil into the lock ring and body threads, and tap it with a wrench or hammer since the vibration helps the oil wick into the threads. Then do this a few more times over a day or 2, and finally war the freewheel one more time and use the pin spanner to start the destructive removal process.
Alternatively, cut your losses and find a good uses 27" wheel and start over.