Cook Pasta: Preheat oven to 375°F (190°C). Cook pasta shells according to package directions until just tender. Drain and set aside to cool.
Make Filling: In a large bowl, combine ricotta cheese, 1 cup mozzarella, Parmesan cheese, sun-dried tomatoes, egg, garlic powder, Italian seasoning, salt, and pepper. Mix until well combined.
Prepare Dish: Spread 1/2 cup marinara sauce on the bottom of a 9×13 inch baking dish.
Stuff Shells: Fill each cooked shell with about 2 tablespoons of the ricotta mixture. Place filled shells in the prepared baking dish.
Add Toppings: Pour remaining marinara sauce over shells. Sprinkle with remaining 1 cup mozzarella cheese.
Bake: Cover with foil and bake for 25 minutes. Remove foil and bake 10-15 minutes more until cheese is golden and bubbly.
Serve: Let rest 5 minutes, then garnish with fresh basil before serving.