Combine pineapple, citrons, peels, cherries, raisins & currants in a large bowl. Add 1/2 c grapefruit juice, mix very well. Cover & let stand overnight. The next day, add the nuts & 1/2 c of the flour to the fruit mixture & mix well. Combine the rest of the flour with the baking soda, salt & cinnamon in a separate bowl, & mix well. In a third bowl, beat the shortening for 30 seconds. Gradually add the sugars, creaming well as you add them. Stir in the cornstarch. Add the beaten shortening & sugars to the fruit. Gradually stir in the flour mixture. Add the almond extract & the rest of the grapefruit juice, stirring well until the mixture is well combined. Preheat oven to 275ºF.  Grease a 10" square cake tin. Line with brown paper & grease the paper. Transfer the cake mixture to the cake tin. Press down gently. Bake for 3 hours. Check after two hours. If the top is browning too quickly, cover with some waxed paper. After it is cooked, remove from the oven & place on a wire rack to cool for 30 minutes. Gently remove cake from the tin & peel away the paper. Let cool on wire racks till completely cooled. Wrap in waxed paper & keep in a cool place. If icing the cake, layer with marzipan & then decorate as desired.