Remove meat from chicken and cut into large chunks. Mix chicken with lemon juice and 1/4 cup mayonnaise in a large bowl. Season with salt and pepper.
Making 1 sandwich at a time, spread 3 slices of toast with mayonnaise. Top 1 slice with 1/2 cup chicken salad; season with salt and pepper. Top with second piece of toast, mayonnaise side up. Add 2 slices of tomato, 2 slices of pickle and a lettuce leaf. Top with a third piece of toast, mayonnaise side down.
Lightly press sandwich down and cut in half diagonally. Secure with toothpicks. Repeat with remaining ingredients.