My concern would be,what made that much rib let go in the first place? It may be a good idea to get a good look underneath before sealing up a problem. In this case I would go the full mile.
Also, the rosin flux will work well in affixing solder to solder, but will not do well in the solder to steel department. One normally tins both pieces of the work, first, using acid flux, then attaches them using the rosin based stuff. In this case, do you really know what you have under there?
Jim