I think Mudhen did something like that or told someone to grind 2 shims down and put them together. I would pm him and see.
I like longer barrels for dove and waterfowl, and I like shorter barrels for upland and turkey
Shorting the barrel will do nothing for you. Barrel length has nothing to do with patterning of a shotgun. Your on the right track with chokes. Cylinder choke will give you the most open pattern. I think IC like PRO said will do just fine. Some one said patterning board. Excellent idea! Benelli's shoot a bit higher then american guns. This lets you see what you are shooting at better.
Nova's will eject the shell out even if you short stoke the gun. It sounds like your fully stoking the gun because the new round comes up from the mag. tube.