If the gun is proofed at 70mm (2 3/4") it can handle the pressure of US 2 3/4" loads. My concern is recoil on the wood and my shoulder so I prefer lighter recoiling loads.
For all of the birds you mention, except pheasants, I would use 2 3/4" 1 ounce #7.5 at 1150 - 1200 fps. I would use the same load for sporting clays and skeet but would use either #7.5 or #8 shot.
I haven't hunted wild pheasants but would increase the load to 1 1/8 or 1 1/4 ounces of #5 or #6 shot with a lower velocity - I'm not a believer in high velocity for lead shot loads.
