What's wrong with (blind) riveting alu (and perhaps using an adhesive too, before riveting)?
It works well on aeroplanes.
Edit: And you can repair it easily in the field. Welds, pfft! ;-)
Edit 2: I see you mean the actual bed. Well, if it's some stringers or some such you think needs welds, you can do well with bolts. For thinner stuff, I still think alu and rivets will be better overall. As for "settling" the truck. That is merely newspeak for "putting more weight on the suspension, and thereby lowering my real load bearing capacity". Think about it: If you put so much weight on your suspension before you actually load you truck up that it makes use of, say, 40 percent of the capabilities of the suspension, and you'd only use, say, 30 percent by using lighter materials, that is a saving of 25%, meaning less wear, less heat, and with a load, it might mean the difference between shocks bottoming out or not.