Preheat oven to 350 degrees F. Cut potatoes in half and place cut side down in a baking dish. Bake until a knife inserted in center goes through easily, about 1 hour. Cool, peel and puree. Combine gingersnap cookies, melted butter and nuts. Press into the bottom of a 9 ½ inch or 10 inch springform pan. Bake for 10 minutes.
In a large mixing bowl, beat 3 packages cream cheese and butter until light and fluffy. Add sugars, sweet potato puree, pumpkin pie spice, vanilla, sour cream and heavy cream. Beat until smooth. Add in 3 eggs, one at a time, blending well after each addition. Pour filling into prepared crust.
In another mixing bowl, combine 1/2 package cream cheese, marshmallow fluff, vanilla extract, 1 egg and flour, until well mixed.
Add drops of the marshmallow mixture on top of the cheesecake and swirl gently with a butter knife.
Fill a pie plate with water and place on the bottom oven rack. Place the cheesecake on the middle rack. Bake for about 1 hour, 10 minutes, or until the center is almost firm. Turn off oven and let cake cool with the door open.