I don't recall ever having seen a 10 ga 00 grade. 20, 16, and 12 but no 10's. It may be a real odd ball.
Don't go any further with your 3 1/2" chamber idea - you will probably destroy the gun & hurt yourself. If you want to shoot ducks with a double go buy yourself a box of 2 3/4" Bismith (provided it's a 12 ga, which I'd bet on) and have fun. That's what I do and it works as well as these 3" shells with steel shot. If it truly is a 10 you can get non-toxic loads that are 2 7/8", but you'll have to order them.
Any "gunsmith" capable of this conversion should know better than to lengthen chambers like this. Find yourself another Smith.