Preheat oven to 350 degrees F. In a small skillet combine 2/3 cup of the sugar with 1/4 cup water and bring the mixture to a boil, stirring and washing down any sugar crystals clinging to the sides with a brush dipped in cold water until the sugar is dissolved. Cook the syrup, swirling the skillet, until it is a deep caramel, pour it into a warm 2-quart glass loaf pan, tilting the pan to coat the bottom evenly, and let the caramel harden.
In a bowl beat the eggs with the remaining 2/3 cup sugar, beat in the pumpkin puree, salt, ginger, cinnamon, allspice, and cream, and pour the custard into the loaf pan. Set the loaf pan in a deep baking pan, add enough hot water to the baking pan to reach halfway up the sides of the loaf pan.
Bake the flan in the middle of a preheated moderate oven for 1 hour and 15 minutes, or until a knife inserted in the center comes out clean. Let the flan cool and chill it, covered, overnight. Run a thin knife around the edge of the loaf pan, invert a platter over the pan, and invert the flan onto the platter. Serve the flan, cut into slices, with the cinnamon whipped cream.