Worst case scenario here. Chromed steel stem stuck in steel fork

I'm restoring a bike for a friend of mine and the headset bearings were shot and it was rusty.

So I decided to remove the handlebar and quill stem. Wasn't budging. The entire thing is rusted shut inside the steering tube.

At first I was hopeful it was just aluminium but 24 hours in caustic soda did absolutely nothing. It's currently soaking in an oxalic acid bath and it has removed a tiny amount of rust near the edges. But clamping it in a bench is just deforming the threading on the fork, nothing more.

Any suggestions what else I can do here? Is drilling going to be my only option?