Lionheart,
You must be brave indeed to attempt such a project. I hope it works out well for you if you procede with it. Since I have no experience with modern damascus barrels, I won't say anything about them, but, I will give you just a couple of thoughts on items 1 and 3 in your questions area.
First, under the heading of the age and general strength of your action, it might be that this 10 gauge is up to the task of handling the 600 Nitro cartridge, but the main concern is the back thrust of tha cartridge heads against the standing breech, as has been mentioned by others already. In my opinion, and that's all it is, if I were going to go to all the trouble of converting this shotgun to ANY large caliber, I would do away with the dogtooth hammers altogether by cutting the noses off and put in the disc set strikers. Sure, it is more work, but in something of this magnitude, I feel it would be a mistake not to do it. You are doing this for yourself anyway, and most of what is required is some extra time and machine work. Buy Ellis' book and follow his instructions on re-doing the strikers, I don't think you will regret doing the extra work later.
Next, if you do decide to try and re-use the top rib, it may be found that it is difficult to remove it, or maybe not, because of the age of this gun. If it is, and you still want to reuse it, cut the barrels away from the mono block section first, then you can probably heat the left over barrels enough to remove the ribs. If that doesn't work, you will have to slice down the tops and bottoms of both barrels with a cutting tool to slice the excess barrels away form the ribs. I clamped a steel strap to the barrels and used a D.A grinder with a cutting wheel to do this part, the strap protects the top rib from the wheel getting into it. Then using a torch and two pairs of pliers or visegrips, you will be able to get the ribs loose from the remainder of the barrels. It is best to have someone hold the torch for you and then you can move the ribs under the flame and apply pressure with both hands simultaneously to get the ribs loose. Apply most of the pulling pressure against the barrel pieces, holding the rib still will doing this. Be extra careful not to get the ribs too hot. They are now much smaller in mass and the metal can be burnt easily if you are not careful. Concentrate the heat out on the edges towards the remaining barrel scraps.
My last thought on your project, and one that nobody has mentiond yet, is that your gun contains a Greener side safety. This may be a problem for you if you decide to restock it. This safety somewhat weakens the head of the stock in an area where it needs to be as strong as possible and, unless you are a well practiced gun stocker, it can be a bit of a problem to inlet into your new stock. If you do restock it, I would suggest a pistol grip type stock, you will have better sighting and recoil control with it. Good Luck on your project! Bob H.