I purchsed my BC used and it already had a Marbles tang sight mounted on the wood. I do not believe there is a steel support under it but the sight feels quite stable and I haven't had any problems with it yet. I've fired roughly 300 38-55's and 200 mid range 45-70's from it and it's holding true.
Personally, if I were to have mounted it myself, I probably would have had a gunsmith make one of those false tangs.
A note to you Kitchawan; If you decide to mount the tang sight, be sure to learn to shoot with your thumb off to the side and not straddled over the top of the stock. After about three or four rounds of good recoiling 45-70 rounds you're sure to have a gouged thumb. My 45-70 was my first experience with a tang sight and needless to say, it was a little bit of a bloody one too. If you're somewhat immune to recoil and don't have any kind of a flinch while shooting, you'll be sure to have one for the rest of the day while wearing that band-aid.