Preheat your oven to 375°F (190°C).
Cook the conchiglie pasta according to the package directions until just tender, then drain and let cool.
In a spacious bowl, mix together the ricotta cheese, chopped spinach, half of the mozzarella, Parmesan cheese, egg, garlic powder, onion powder, Italian seasoning, and a sprinkle of salt and pepper until well blended.
Pour a cup of marinara sauce into the bottom of a 9x13 inch baking dish, spreading it out evenly.
Carefully fill each conchiglie with the ricotta and spinach filling and arrange them in the baking dish atop the sauce.
Drizzle the remaining marinara sauce over the stuffed shells, then top with the rest of the mozzarella cheese.
Cover the dish with aluminum foil and bake for 25 minutes.
Remove the foil and continue baking for another 10-15 minutes, or until the cheese is melted and golden brown.
Allow the dish to cool slightly before serving, and garnish with fresh basil leaves if desired.