Get some Liquid Wrench. Try to elevate the rear end slightly so the bolt is out of level.
After spraying on the Liquid Wrench, tap the bolt lightly for a minute or so. Re-apply, tap for another minute or so.
Soaking alone doesn't cut it with penetrating oil, it needs the slight vibration to allow the solvent to cut through and displace the corrosion.
Be patient and don't snap it off. It will eventually free up.
Heating the bolt will only make it larger and harder to get out. If anything, heat the surrounding threads. I would only do this with a small butane pen-tourch that has a ultra-small flame that you can control. Heat only the nut that the bolt threads through.