In a shallow dish combine lime juice, garlic, cumin, and oregano. Add steak, cover and refrigerate 8 hours to overnight.
Pre-heat the oven to 350 degrees. Cook pasta shells according to package directions. Drain and rinse in cold water, set aside. Heat and lightly grease an outdoor grill or indoor grill pan. Grill steak, onions, and peppers for 6-8 minutes (or until the steak has reached the desired doneness) turning once. Remove the steak and vegetables and cool slightly. Cut the steak into thin slices and chop the green pepper and onion. Place steak, green pepper, and onion into a medium bowl. Stir in sour cream, 2 tablespoons of Mexican cheese blend and cilantro.
Stuff the cooked pasta shells with the meat mixture. Arrange shells on a lightly greased 8-inch baking dish. Pour salsa over the shells.
Cover with foil and bake for 25 minutes or until heated through. During the last few minutes or cooking remove the foil and top the shells with leftover Mexican cheese blend. Remove from the oven and serve over a bed of shredded lettuce.