I have been doing a lot of cantilever post repair lately. Particularly on polo bikes. I have also been putting cantilever mounts on bikes intended for caliper brakes. Here are a few pictures of the process. I sometimes TIG weld them on and sometimes I brass braze them. If I'm mounting new bosses I stick to brass because it looks nicer and they can be heated and removed which is easier than with TIG because they have to be cut off with a hack saw. If the original boss is welded then I go ahead and TIG the replacement in place. I use a modified rack stay to keep the posts aligned.