Preheat the oven to 375°F (190°C).
Cook the jumbo pasta shells according to package instructions until al dente. Drain and set aside to cool slightly.
In a large bowl, combine ricotta cheese, chopped spinach, 1/2 cup mozzarella cheese, Parmesan cheese, egg, garlic powder, Italian seasoning, salt, and pepper. Mix until well combined.
Spread 1 cup of marinara sauce evenly on the bottom of a 9x13 inch baking dish.
Stuff each cooked shell with the ricotta and spinach mixture, then place them in the baking dish on top of the marinara sauce.
Pour the remaining marinara sauce over the stuffed shells, then sprinkle the remaining 1/2 cup of mozzarella cheese on top.
Cover the baking dish with aluminum foil and bake for 25 minutes. Remove the foil and bake for an additional 10 minutes, or until the cheese is bubbly and golden.
Let cool for 5 minutes before serving. Garnish with fresh basil if desired.