Place each chicken breast half between 2 sheets of heavy-duty plastic wrap; flatten to 1/2-inch thickness, using a meat mallet or rolling pin. Combine chicken breast halves and 1/2 cup dressing in a heavy-duty zip-top plastic bag; seal bag securely, and marinate in refrigerator at least 8 hours.
Remove chicken from marinade, discarding marinade. Place chicken on grill rack coated with cooking spray; grill, covered, 4 minutes on each side or until done. Top with cheese, and grill 1 minute or until cheese melts. Coat bread with cooking spray; grill, covered, 1 minute on each side or until toasted.
Toss lettuce with 1 tablespoon dressing. Top bread slices evenly with lettuce and tomato. Place a chicken breast over each. Sprinkle with basil and pepper. Serve immediately.