The Stevens Model 225 was manufactured between 1906 and 1910. Production was short lived and further complicated by the fact that Stevens changed their method of manufacturing barrels, going form a mono-block system to a chopper lump system, which they called Demi-Bloc shortly after the Model 225 was introduced. I do not believe that the mono-block barrels will interchange with the Demi-Bloc barrels on the Model 225.
Selling your Model 225 for parts (Model 225 parts are scarce) and investing in a complete double barrel is the practical approach to your problem.