Aluminum can be direct welded but it's harder to do and many shops will resort to TIG as it gives cleaer results. Once you alloy magnesium more than 25% with anything ti's tendancy to burn like a torch drops ways off. I've done most Aluminum alloys using TIG and the preferd torch is the single arc (Arc to work) and the prefered gas either argon or helium. When done with He the process gets called heliarc. I've done both. In both cases you need a TIG rig and inert gas. Many welding shops worth their salt can do this. But often charge by the hour for a one off. It helps of you've done all the prep so they can hook up and go. A good TIG weld should require minimal refinishing to look good. Don't forget a lot of trucks and trailers these days have a lot of Aluminum and they do get repairs so the skills are out there. Due to costs, I'd seriously first try to fine a stripped chassis first. Allison