Bring a large pot of salted water to a boil and cook the jumbo pasta shells until just tender. Drain and set aside to cool slightly.

In a mixing bowl, combine the ricotta cheese, drained spinach, half the mozzarella, the Parmesan, egg, garlic powder, dried basil, salt, and pepper. Stir until evenly mixed.

Spread 1 cup of marinara sauce in the bottom of a baking dish. Spoon the ricotta mixture into each cooked shell and place them in the dish.

Pour the remaining marinara sauce over the stuffed shells and sprinkle with the rest of the mozzarella cheese.

Bake at 375°F for 25 minutes, or until the cheese is melted and the sauce is bubbling around the edges.