In a large bowl, dissolve yeast in warm milk. Add the pumpkin, sugar, butter, salt and 4-3/4 cups flour. Beat until smooth. Stir in enough remaining flour to form a soft dough (dough will be sticky).
Turn onto a floured surface; knead until smooth and elastic, about 6-8 minutes. Place in a greased bowl, turning once to grease top. Cover and let rise in a warm place until doubled, about 1 hour.
Punch dough down; divide in half. Roll each portion into a 12-in. x 10-in. rectangle. Combine the flour, brown sugar, cinnamon and allspice; cut in butter until crumbly. Set aside 1 cup.
Sprinkle remaining streusel over dough to within 1/2 in. of edges; press down lightly. Roll up jelly-roll style, starting with a long side; pinch seams to seal.
Cut each into 12 slices. Place cut side down in two greased 13-in. x 9-in. baking pans. Sprinkle with reserved streusel. Cover and let rise until doubled, about 30 minutes.
Bake at 375° for 20-25 minutes or until golden brown. Meanwhile, combine the confectioners' sugar, vanilla and enough milk to achieve desired consistency; drizzle over rolls. Serve warm. Yield: 2 dozen.