A Propane torch is the way to go....adjust it to it's highest setting and direct the tip of the flame cone to the area you want heated. ( The tip of the cone is the hottest part of the flame). Do not heat the bolts but heat the surrounding aluminum. The large aluminum parts will absorb a lot of heat so you must be patient. Do not force anything. When the aluminum is Totally hot, the bolts should loosen. You should not need any more force than can be provided by a normal open end wrench. If it doesn't move, you did not heat it enough or you were heating too far from the area surrounding the bolt. Heating these big pieces of aluminum will usually require at least 3 minutes or more and is best done in still ( no wind ) conditions. Success will depend on technique and whether the motor was used in fresh or salt water and whether the previous technician used anti seize or not. Good Luck!