Great old gun but several issues. It likely started out with 2 5/8" chambers. Shooting modern plastic 2 3/4" shells is unlikely to increase chamber pressure significantly BUT your wood is now more than 100 years old. To prevent cracks along the side plates or even a cracked wrist, it would be wise to limit your shells to 1 1/8 oz (no Fiocchi Golden Pheasant pheasant loads!) After the season, it would also be a good idea to have an experienced doublegun smith check and clean the action, possibly reinforce the wood around the side plates, and measure your choke constriction (likely M/F but by your description possibly IC/F.) With proper care you might be able to pass the gun on to your grandchild!