Preheat oven to 375°F (190°C).
Cook jumbo shells according to package instructions, then drain and set aside.
In a large bowl, mix ricotta, mozzarella, Parmesan, cooked spinach, egg, garlic powder, salt, and pepper until creamy and well combined.
Spread half of the marinara sauce into the bottom of a baking dish.
Fill each pasta shell with the ricotta mixture and place them in the dish, seam-side up.
Spoon the rest of the marinara sauce over the shells.
Sprinkle extra mozzarella and Parmesan on top.
Cover with foil and bake for 20 minutes.
Remove foil and bake another 10 minutes until bubbly and golden.
Top with fresh basil and serve warm.