Preheat an oven to 350 degrees F (175 degrees C). Lightly grease a 9x13 inch baking dish.
Fill a large pot with lightly salted water and bring to a rolling boil over high heat. Once the water is boiling, stir in the shell pasta, and return to a boil. Cook the pasta uncovered, stirring occasionally, until the pasta has cooked through, but is still firm to the bite, about 13 minutes. Drain well in a colander set in the sink.
Heat a large skillet over medium-high heat. Cook and stir ground beef, onion, and green pepper until browned. Drain off excess grease. Stir in pepperoni, tomato sauce, tomato juice, oregano, celery salt, onion powder, garlic salt, basil, red pepper flakes, and cooked pasta. Pour into the prepared baking dish and cover with aluminum foil.
Bake in the preheated oven for 20 minutes. Remove casserole from oven and sprinkle with mozzarella cheese. Return to the oven and bake until the cheese melts, about 5 minutes.