Preheat the oven to 325 and set the oven rack in the middle position. Spray 10" bundt pan with non-stick cooking spray and dust with flour.
In medium bowl, whisk together the flour, baking soda and salt. Set aside.
In the bowl of an electric mixer fitted with paddle attachment, cream the butter and sugar on medium speed until light and fluffy, 3-4 minutes. Scrape down the sides of the bowl, then beat in the eggs one at a time, beating well after each addition. Scrape down the sides of the bowl again.
In another bowl, combine the buttermilk, lemon zest and lemon juice.
With the mixer on low speed, beat in one-quarter of the flour mixture, then one-third of the buttermilk mixture. Beat in another quarter of the flour, then another third of the milk mixture. Repeat with another quarter of the flour and the remaining milk mixture. Finally, beat the remaining flour mixture. Scrape down the sides of the bowl, and give a quick mix to make sure all the ingredients are well incorporated.
Spoon the thick batter into the prepared bundt pan and smooth with a rubber spatula. Bake 1 hour and 5 minutes, or until a cake tester comes out clean.
Cool the cake in the pan for 10 minutes on a rack. Invert cake. When cool, add the icing.
Frosting: Combine the crushed pineapple with juice and instant pudding mix (no milk) in bowl and beat 2 minutes at medium speed. Let sit 5 minutes. Thoroughly fold in cool whip. Frost cake and chill at least 2 hours.