In a large bowl, stir together the flour, 1/2 cup sugar, and salt. Cut in 1 cup of margarine until mixture resembles coarse crumbs. Sprinkle the mixture evenly over the prepared pan, and press in firmly.
While the crust is baking, prepare the filling. In a large bowl mix together the eggs, corn syrup, 1 1/2 cups sugar, 3 tablespoons margarine, and vanilla until smooth. Stir in the chopped pecans. Spread the filling evenly over the crust as soon as it comes out of the oven.