pacomb,
This work is beyond my abilities, but I am given to understand the best way to do it is with an oven, especially made for the purpose. The joints must be cleaned, flux and solder applied, then the ribs have to be wired and held to the barrels with soft iron wire, blocks/wedges and then heated to the flow point of the solder in the oven. The main part of the ribs are traditionally soft soldered on. There is usually a section over the lugs and chamber area that is silver soldered. The soft solder is usually 50/50 Tin/Lead, or some may have used 60/40. Rosin flux was traditionally used to avoid corrosion caused by being unable to clean acid flux from between the barrels and ribs. Soft Iron wire is used as "tie wire" for reinforcing rods in concrete construction and is available at building supply stores. My limited experience is only with very short sections of loose rib, but it enough to indicate that it would be difficult to get enough heat, evenly, on full length ribs with a handheld torch. If it is not necessary to resolder the entire length of rib, you should wire and block the whole thing to prevent other sections from coming loose. I hope this helps.
