In a large bowl, stir together flour, granulated sugar, and brown sugar. Using a pastry blender, cut in butter until mixture resembles coarse crumbs. Set aside 1 cup of the crumb mixture.
Press remaining crumb mixture onto bottom and halfway up sides of ungreased 2-quart rectangular baking dish.
In another large bowl, stir together pumpkin, sweetened condensed milk, eggs, cinnamon, salt, and allspice. Pour into crust-lined baking dish.
Stir pecans into reserved crumb mixture. Sprinkle pecan mixture over pumpkin mixture.
Bake for 50 to 55 minutes or until knife inserted near center comes out clean. Cool in pan on wire rack. Chill in refrigerator for up to 2 hours. Store covered in refrigerator.
