Preheat oven to 350 degrees F (175 degrees C). Grease a 9x12 inch baking dish. Bring chicken broth and salt to a boil in a large saucepan over high heat. Stir in the grits, return to a simmer, then reduce heat to medium-low, and continue cooking for 20 minutes, stirring frequently. Meanwhile, melt the margarine in a skillet over medium heat. Stir in the green onions, green pepper, and garlic; cook until the peppers have softened, about 5 minutes. Stir in the shrimp, and cook until they begin to firm. Stir the Monterey Jack cheese, 3/4 cup Cheddar cheese, shrimp and vegetable mixture, canned tomatoes, and black pepper into the grits; pour into prepared baking dish and sprinkle with remaining 1/4 cup Cheddar cheese. Bake in preheated oven until the cheese is bubbly and beginning to brown, 30 to 45 minutes.