Why not break the corrosion bond by pulling it further into the steer tube? Rig up some sort of affair that will hook onto the underside of the fork crown and won't snap under the considerable tension you're going to be putting on it. Also, this way, you won't strip out the threading in the aluminum.

Be careful. I cracked the brazing into the fork crown trying to get a stem out by twisting it, which was an expensive mistake. I came up with /\/\ that plan during a late-night after-action review over beers, but never did make up a tool.