Using mixer and large mixing bowl, put in cake mix, 1 box of instant pudding, eggs, oil, 7Up, and nuts (if desired). Beat well until smooth (3-4 minutes at medium speed). Pour batter in greased and floured long cake pan 13x9x2. Bake at 350 degrees for 45 minutes. Cool cake.
In a mixing bowl, add 2 packages of Dream Whip and 1 1/2 cups of milk. Whip until it forms peaks. Add other box of pistachio pudding and mix until smooth.
Spread frosting on cake. Sprinkle coconut over the entire top. Refrigerate overnight before serving.