Preheat the oven to 375°F (190°C).
Cook the pasta shells according to package instructions until al dente, then drain and rinse under cold water.
In a mixing bowl, combine ricotta cheese, mozzarella, Parmesan, pepperoni, oregano, garlic powder, salt, and pepper.
Spoon about 2 tablespoons of the cheese mixture into each shell, packing them gently.
Spread half of the marinara sauce in the bottom of a 9x13 inch baking dish.
Arrange the stuffed shells in the dish, then cover them with the remaining marinara sauce.
Sprinkle additional mozzarella and Parmesan cheese on top, if desired.
Cover the dish with aluminum foil and bake for 25 minutes. Remove the foil and bake for an additional 10-15 minutes until the cheese is bubbly and golden.
Let it cool for a few minutes before garnishing with fresh basil and serving.