First off, Duncan Hines doesn't make commercial sized cake mixes for a bakery. I worked for a caterer and she ONLY liked Duncan Hines cake. Yes, she bought cases of DH cake mixes from Wal-Mart and whoever has sales going that week. Even though we were making 20+ cake layers at a time, each cake mix was opened one at a time.
I think you can buy pudding mix in commercial sized bags, you would have to see how many cake mixes to the weight of the commercial sized bag of pudding mix and go from there.
As for converting, you can find commercial books with recipes in weights that convert to large scale baking, or make your own. Get a good scale and write each and every weight, even water and eggs in ounces or grams (choose one) and go from there. Then you can easily increase the amount based on the weights easier than measuring all the ingredients with cups. Also, keep track of the size mixing bowl you use. That same caterer had a 5qt, 12qt, 20qt, 30qt and 80qt mixing bowl to choose from. Many new bakers got confused and used the wrong bowl, so keep that on your recipe for reference as well!!!
I am pretty good at converting recipes, up and down. Every test in Culinary School had 10 questions from weight/measurements/increasing/decreasing and so it was really drilled into our heads!!