Hevi 3-1/2" 2.25 #6 is certainly a good place to start. Do some pattern testing and you might find the Hevi 3-2-6 version might pattern equally well, or even better.
Shotguns are picky, and more shot isn't always better. I use an IC BDS .660" in a Mossberg 535 with a 20" tube. With Hevi 3-2-7 shot, it's putting 240 pellets in a 10" circle at 40 yards.
My search for a choke/ load combo is over... I finally have a solid 40 yard Turkey killer.