this is how I removed my stuck quill stem.
cut the stem below the bars, remove the forks, wrap the forks in a towel and put them in a vise. Get a friend to heat the steerer tube around the stem while you pull and rotate the stem with a pair of multigrips. If the steerer tube is bonded to the forks keep the towel wet. You may have to remove the crown race before putting the forks in the vise. When the stem has been removed let the steerer tube air cool (dont dunk it in water). Clean with a wire brush.
Make sure you thoroughly grease the new stem and do it a couple of times a year