Reclamp the other bolts to factory torque. Loosen the offending bolt using a 6 to 8 foot long steel tube, such as scafolding rods, or a piece of thick boiler/exhast tubing. Engauge it over the breaker bar and socket, and give it a sharp blow. If that won't shift it, I'll consider celibacy.
The socket can be a 12 point socket or a 14 mm (soft hammerd on), rather than the 9/16, if you have issues with it hanging on. It worked for my old 200. A soak in petrating oil in that valley where the exhast heater stove and carb is will help if you want to wait a day.