Add pasta and cook for 8 to 10 minutes or until al dente. Drain and cool. Meanwhile, in a large skillet brown beef on medium heat. Drain and cool.
Stuff shells with mixture.
Lightly grease a 9x13 inch baking dish. Pour enough spaghetti sauce in dish to cover the bottom. Place stuffed noodles in the dish. Cover with spaghetti sauce and top with the remaining 1 cup mozzarella cheese. Bake in preheated oven for 50 minutes.