Preheat your 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 mixing bowl, combine the ricotta cheese, pumpkin puree, 1/2 cup of mozzarella cheese, Parmesan cheese, garlic powder, Italian seasoning, salt, and black pepper. Mix until well combined.
Spoon the pumpkin filling into each cooked pasta shell, filling them generously.
Spread half of the marinara sauce on the bottom of a 9x13 inch baking dish. Arrange the stuffed shells in a single layer on top of the sauce.
Pour the remaining marinara sauce over the stuffed shells, then sprinkle the remaining mozzarella cheese on top.
Cover the baking dish with aluminum foil and bake for 25 minutes. Remove the foil and bake for an additional 10-15 minutes, or until the cheese is bubbly and golden.
Let the dish cool for a few minutes before serving. Garnish with fresh basil leaves if desired.