Preheat oven to 350 degrees F. In medium bowl, combine 1 1/2 cups flour and 1/2 cup confectioners' sugar; cut in margarine until crumbly. Press onto bottom of lightly greased 13x9-inch pan; bake 15 minutes.
Meanwhile, in a large bowl, combine eggs, granulated sugar, baking powder, lemon juice, and remaining 3 Tbsp. flour; mix well. Pour over baked crust.
Bake for 25 minutes or until lightly browned. Cool, then cut bars and sprinkle with confectioners' sugar.