In a large skillet over medium heat, mix ground beef and sausage. Cook and stir until evenly browned. Mix in salt and pepper, pumpkin pie spice, and brown sugar. Drain meats, and mix in chicken stock and rice. Cover, and cook 25 to 30 minutes, until rice is tender. Preheat oven to 400 degrees F (200 degrees C). Remove and reserve top of pumpkin. Scoop out seeds and stringy pulp. Mix cream of celery soup, cream of mushroom soup, mushroom stems and pieces, and green beans into the beef and sausage mixture. Spoon mixture into the pumpkin, and replace pumpkin top. Place pumpkin on a large baking sheet, and bake 1 hour in the preheated oven, or until pumpkin is tender. Scoop out portions of filling and parts of the cooked pulp to serve.