All I can suggest is blood, toil, sweat, and, perhaps, some tears! Use an open end 9/16" wrench and move those nuts 1/16 of a turn at a time -- whatever -- until they come loose. Be sure to chase the threads of the stud and nut, and use plenty of grease when you reassemble so that the job goes quickly in the other direction. I assume you've removed the carburetter and the heat shield, as those will certainly slow you down.