Preheat the broiler to high.
Bring a large pot of water to a boil. Add the shells and cook according to package directions. Drain, reserving some of the pasta water and set aside.
Meanwhile, arrange the tomatoes, cut-side up, on baking sheet and grill until wrinkled and soft, 4 to 5 minutes. Combine the breadcrumbs, 1 teaspoon of the oil and the Italian seasoning. Sprinkle over the tomatoes. Continue to grill until golden brown, about 1 minute.
Heat the remaining 1 tablespoon of oil in a medium saucepan over medium heat. Add the flour and garlic, stirring until the flour turns light brown, about 1 minute. Add the pumpkin puree and evaporated milk, bring to a simmer, stirring, and cook until sauce just begins to bubble and thickens, 4 to 5 minutes. Stir in the Cheddar, mozzarella and Parmesan, 1/2 teaspoon salt and some pepper. Stir until melted, about 1 minute.
Add the spinach and stir until wilted, about 1 minute. Add the cooked pasta and stir until coated, thin out with the reserved pasta water as desired. Divide the mac n cheese, tomatoes and grapes among four plates.