Preheat oven to 250°. Place popcorn and nuts in a greased roasting pan; set aside. In a large heavy saucepan, combine brown sugar, corn syrup, butter and salt. Bring to a boil over medium heat; cook and stir 5 minutes.
Remove from heat; stir in baking soda (mixture will foam). Quickly pour over popcorn and mix well.
Transfer to two greased 15x10X1-in. baking pans.
Bake 1 hour or until dry, stirring every 15 minutes.
In a microwave, melt each chocolate separately, adding 1 teaspoon shortening to each 1/2 cup chips; stir until smooth. Drizzle over popcorn mixture; let stand until set.
Break into clusters. Store in an airtight container. Yield: 5 quarts.