Well fenders range from 18-19-20 gauge, running boards 16-18, I can't say you can weld s/s in there but that is not how they are done at restoration shops. Gas welded patches of the same gauge metal. Gas (oxy-acetylene) leaves the joint the softest of any of the welding processes. The softness of the joint is good to have as part of the welding process has shrinking of the metal "every" weld joint will have some shrinkage. Now with gas welding it can be hammered smooth and very little or no filler needed. If you like to sling some mud just filler up the weld joint. There are a couple of good booklets one is the Key to Metal Bumping, and there is a good Gov't/military version too.
I've been making patch panels for well showing my age now about 40 years. If you would like more info I can hook you up with some websites and some publications.
