What's the repair?
body is rather thin. TIG is the best process to weld thin aluminum.
Roof is thick and can be done with a MIG or TIG
4043 is a good choice of filler material on the HMMWV body
With aluminum it's all in the prep. you need clean, bare metal for a good weld.
Aluminum weld is generally weaker then the base material. That is two pieces welded together is weaker then 1 piece of the same dimension.
If you're filling holes, that doesn't really matter.
If you're doing something structural, the weld, even if done well, is going to the be the weakest point.
This is different then most steel fabrication. With steel, the filler mater is often of higher strength compared to the base when welded properly.
One of the reasons riveting is so common with aluminum sheet construction & why you often see sheet aluminum repaired by sandwiching another layer over a flaw.