In a large saucepan, melt 4 tablespoons butter. Stir in the flour and salt until smooth. Gradually add milk. Bring to a boil; cook and stir for 2 minutes or until thickened. Remove from the heat.
Stir in the cheeses until melted. Add potatoes; stir gently to coat. Transfer to a greased 2-1/2-qt. baking dish. Sprinkle with lemon juice.
In a small skillet, melt the remaining butter. Add bread crumbs; cook and stir over medium heat until lightly browned. Sprinkle over potato mixture. Bake, uncovered, at 350° for 40-45 minutes or until hot and bubbly.