If the nut is really that frozen I would just drill some small holes on the flat sides of the nut 180* from each other and cold chisel it off, just be careful not to drill too deeply and damage the threads on the shaft.
The nut is stupid cheap and if you think the nut is stuck, wait til' you go to pull the impy.....

Slow and steady wins the race...But sometimes you need to get to the track a little faster...

GT