Don't mess about with electric impact drivers, they are more for use with masonry bits, not loosening screws that have been done up forever. Use the manual sort, with a bit that fits well…make one if you have to by grinding the end of one. My impact driver will take a half inch socket, which means you can use the big screwdriver bit that comes with some socket sets. You should put a twisting force on the impact driver before you thump it.
Heat is actually fine to use. You want something like big propane burner, not oxyacetylene which is too local. Bring the job up to moderately hot, eg spit fizzes on it, but not red heat…just a good black heat is plenty. Try the impact driver while it is hot. If no joy, let it cool down and try again while cold. Repeat as necessary. Try leaving it soaking with penetrating oil, take care of course if you reheat it later. I have had screws that resisted this sort of thing for days…everytime I walked past I would give the screw a tap or two on the head and then try the impact driver, and once or twice a day I would try the heat. Quite often when they eventually give up and come out, it is a much lighter thump on the impact driver than you tried before. The repeated heating and cooling helps break the bond between the screw and the hole, penetrating oil will slowly work its way in, and some of the earlier thumps may have had an effect that was not noticeable at the time.
John