In a heavy saucepan, mix the 1/3 c sugar and cornstarch. Add milk slowly and mix well. Stir over medium heat, until mixture reaches a boil.
Separate eggs; beat egg yolks in bowl. Add about ½ cup of the hot pudding mixture to the egg yolks, a tablespoon at a time, beating well each time. Add egg mixture into pudding mixture. Allow to boil, stirring constantly 2 minutes. Remove from heat. Add margarine, vanilla, and 1 cup flaked coconut.
Beat egg whites to soft peaks. Add cream of tartar and sugar; beat to stiff peaks. Pour pudding mixture into pie shell. Top with meringue, sealing to edges. Sprinkle 1/3 cup coconut on top.
Bake in 350 degree oven for 7-10 minutes, watching carefully and rotating pie if needed. Remove promptly when meringue peaks are lightly browned.