A buddy and I were going to test weld a bulkhead, but haven't got to it yet. I think the chances of welding the cast aluminum is pretty low. The impurities in the casting is likely way to high. I agree with Aaron. It is best to just replace it. It may only be a few hours, but it is a lot of rivets. When I did mine I only had a hand rivet gun and it took me a total of 6.5 hours but that included breaks to rest the arms. A power rivet gun would probably be worth the investment for the job and save a significant amount of time and effort.
Jack