Heat oven to 400°F. Generously grease 13x9-inch pan with shortening; lightly flour; or spray with cooking spray. In small bowl, stir together flour, salt and baking powder; set aside.
In large bowl, beat powdered sugar, granulated sugar and butter with electric mixer on medium speed, scraping bowl occasionally, until blended. Beat in 1 egg at a time until blended. Beat 1 minute longer. On low speed, beat in flour mixture alternately with lemon juice, beginning and ending with flour mixture, until well blended. Stir in lemon peel and 1/2 cup pecans. Pour into pan.
Bake 22 to 27 minutes or until golden brown. Meanwhile, in small bowl, beat frosting ingredients on low speed until smooth and spreadable.
Spread frosting over warm bars. Sprinkle with 1/4 cup pecans. Cool completely in pan on cooling rack, about 1 hour. For bars, cut into 6 rows by 4 rows. Garnish with lemon slices.