The best advice that I can give is go to http://allaboutshooting.com/ and you can find a guy by the name of clark bush. I have been talking with him over the last few weeks and he has an un-limited amount of information. He is a former world record holder in still target shotgun shooting in the mens division and his wife is a world record holder in the womens division. He also tests and patterns new shotshells for shell manufactures and recommends what the best shell and choke is the best for a particular brand of gun. He has shot every shotshell and choke combination imaginable...some that I haven't ever even heard of. His email is at the website and he can give you a tremendous amount of help, free of charge or obligation. From my talks with him, 3" shells will almost always pattern better than 3.5" and #6 shot has more than enough energy to knock down a bird from any "ethical" range. I would definitely talk with him and see what he recommends for your gun.

In the past, I have shot a 3.5" #5 shot Federal with flight control wad out of a Remington 870 SPS-T with thumbhole using a Primos Jellyhead choke tube. I have had decent patterns out past 40 yards. I am always trying to improve my pattern and I am going to try a few other combos this year, starting out with a Strut Stopper Xtreme choke using 3" Winchester HV #6 shot. Your gun will shot differently than mine, so you will have to do some experimenting with it.

Best of luck.