Preheat the oven to 350°F (175°C).
Cook the jumbo shells in a large pot of boiling salted water according to package directions until al dente; drain in a colander and set aside.
In a large skillet over medium-high heat, cook the ground beef until no longer pink, breaking it up with a spatula; drain excess fat.
Reduce heat to medium, add the taco seasoning and cream cheese to the beef, and stir until the cream cheese is melted and the mixture is well combined.
Stir in the drained corn and drained, rinsed black beans until evenly distributed; remove from heat.
Spread the salsa evenly across the bottom of a 13 x 9 x 2-inch baking dish.
Stuff each cooked shell with a spoonful of the meat and bean mixture and arrange the stuffed shells in the baking dish on top of the salsa.
Pour the taco sauce evenly over the arranged shells; cover the dish with aluminum foil and bake for 30 minutes.
Remove the foil, sprinkle the Monterey Jack and cheddar cheeses over the shells, and return the dish to the oven uncovered for about 10 minutes until the cheese is melted and bubbly.
Top with green onions, sour cream, olives, and tomatoes if desired, then serve.