Actually, the SAAMI specifications call for the same pressure for the 3" and 2 3/4" twelve gauge, so the gun isn't likely to fly apart with the use of the longer shell, but I suspect that the barrels may be Damascus and not even suitable for modern 2 3/4" ammo. As far as an insert in one of the shotgun barrels. The problem will be the firing pins being too large in diameter for the pressures involved. The primers will flow back into the firing pin holes and lock the gun up, and could even perforate, dumping high pressure into the action and head of the stock.
If this were my gun, I would limit its use to small game and varmints within its capabilities.