Preheat the oven to 325 degrees F. Bring a large saucepan of salted water to a boil. Add the macaroni and cook until slightly al dente, about 8 minutes. Drain and set aside to keep warm.
Whisk the eggs in a large bowl.
Add the Velveeta, 1 stick melted butter and 2 cups of the half-and-half to the large bowl of eggs. Add the warm macaroni tossing until the cheese has melted and the mixture is smooth. Add the remaining half-and-half, 4 cups of the sharp yellow Cheddar, 1 cup of Parmesan and the remaining shredded cheeses, and salt and pepper, tossing until completely combined in the large bowl.
Pour the mixture into buttered baking dishes (approximately 3 (3-quart) baking dishes). Mix together remaining 2 cups of grated Cheddar, 1 cup Parmesan, and bread crumbs. Sprinkle casseroles with the bread crumb mixture. Drizzle melted butter on top. Bake covered with foil for 35-40 minutes. Uncover and bake another 15-20 minutes more.
___ Optional Items ___ (Add before baking).
Add in two cups cooked broccoli cuts and 2 cups cooked diced chicken breast.
Add in 2 cups cooked crumbled bacon and 1 cup chopped scallions.
Add in 2 cups diced cooked ham.
Add 2 cups chopped tomates and 1 cup chopped fresh basil.
