This will be my first duck hunt with interchangable choke tubes (used to have a Remmington 870 Express). I have upgraded to a Benelli M1 Super 90, 28″ barrell with several choke tubes. The hunting situation will be as follows: 35 yards maximum distance between the ducks and my gun, and no-lead shot is required.
Also is there a recommended size shot (ex. BB, T’s, 2, 4, etc.) with this gun that anyone might know of? I hunt in Southern Michigan FWIW. I would definitely use your modified choke. I assume that you are hunting over decoys and you will get a great shot which will call for a pattern that will not put too many bbs in the meat but will drop the bird. I always use modified and have never had any trouble. It might be a different situation if you have passing shots at a greater length.
Just make sure that you wait long enough for the ducks to cup in to your decoy spread. Steel shot patterns much tighter then lead so I like to use improved- cylnder for ducks and modified for geese. I use 1 & 1/4 oz. of 2′s or 1 & 3/8 oz. of 1′s for ducks. T’s only have about 56 pellets per shell which is way too few for ducks IMHO. I don’t even like them for geese because too often you only put 1 or 2 pellets in the bird and it flies off to die somewhere.