I believe there are special walkers for corn and soybeans, and standard ones for cereals. you can harvest corn, soybeans and wheat on the wheat walkers, but you got reduced capacity in anything but wheat.
I also have a 7720 II. One of the best mods I've done is to replace the top chaffer with a fixed air foil. Mine came from St John's Welding, however, I think he may have stopped making them or sold the patent to someone else.
I run corn, soybeans, and wheat. I really like the job it does and was a real improvement in capacity. The increased air and upwards angle have removed this area as a limiting factor for capacity in my combine. I'd stay way from the adjustable air foils. Not sure, but I think Peterson makes a good fixed one now. Good Luck.