As klhanson said, if it was originally soft soldered, you will have to remove the old filler 100% before you can weld or braze it. From Ottos first pic, it looks like his was brazed. You can re-braze it if that was the original. Brazing is just like soldering only at a little higher heat for the chosen filler. Sil-Fos brazing rod is available in different silver content and is pretty forgiving, you just need a flux designed for steel, or use silver solder with steel flux. If it was mine, I wouldn't use solder just due to the type of joint, it isn't well supported, I would braze it.
