Preheat oven to 350 degrees F. Gather two 9x13 baking dishes and a large plate or baking sheet and set aside.
Bring a large pot of salted water to a boil. Cook the jumbo shells in the water until they are al dente, according to package instructions. Drain the shells, rinse with cold water, and lay them out on the large plate or baking sheet to cool.
While the shells cool, mix together the pumpkin puree, ricotta cheese, 3/4 cup of the parmesan cheese, beaten egg, garlic, sage, salt, pepper, nutmeg, and cinnamon in a large mixing bowl.
Divide the marinara sauce evenly between the baking dishes and spread it out with a spoon or rubber spatula. Take a shell in your hand and try to gently pop it open. Use a small spoon to fill the shell with about 2 heaping tablespoons of the pumpkin ricotta filling (you can use a piping bag instead if you have one). Place the filled shell into one of the baking dishes, and repeat until all of the shells have been stuffed and the baking dishes are full. Aim to have the same amount of shells in each dish so that they can be snuggled next to each other while they bake. Sprinkle the rest of the parmesan cheese on top of the shells.
Cover each dish with foil, transfer to the oven, and bake for 30 minutes, rotating the dishes halfway through. Remove the foil, and return to the oven to finish baking uncovered for 10 more minutes. Enjoy warm!