Be sure to soak all the bolts down with good penetrant like PB Blaster,, Kroil or something of that grade. Using a GOOD boxed end wrench, try to tighten just a little at first then loosen. If it starts to get tight when loosening, spray a little more penetrant on the bolt threads and re-tighten a little then back it off . If it gets tight again, let it sit and cool down as the bolt/nuts will get hot from friction and cause the bolt/stud to break.
If that process does not work well, use heat and let cool, use heat again and let cool again. It will not hurt to heat and let it cool a few times.
Look here for a whole bunch of great tips
http://www.stovebolt.com/helpme.htm
