Prepare and roll out pastry. Line a 9-inch pie plate with pastry; trim and crimp edge as desired. Set aside.
Remove skins from grapes by gently pressing each grape between your fingers. The skins will slip off easily. Set skins aside. In a large saucepan bring grape pulp to boiling. Reduce heat and simmer, uncovered, for 5 minutes. Sieve the pulp to remove the seeds. Add the grape skins to the pulp.
In a large mixing bowl stir together the 3/4 cup sugar, the 1/3 cup flour, and salt. Stir in the grape mixture, melted butter or margarine, and lemon juice. Pour mixture into the pastry-lined pie plate. To prevent overbrowning, cover the edge of the pie with foil. Bake in a 375 degree F oven for 20 minutes.
Meanwhile, stir together the 1/2 cup flour and the 1/2 cup sugar. Cut in the 1/4 cup butter or margarine until mixture resembles coarse crumbs. Remove foil from pie. Sprinkle crumb mixture over pie. Bake about 25 minutes more or until topping is golden. Cool on a wire rack. Makes 8 servings.