I have to agree TIG filler rods have additional metals in them. But I have heard that using thing strips of parent metal on Land Rover repairs is common as the old series ones in particular were a weird alloy.
Stress corrosion cracking would not be a concern as it is a service induced defect requiring stress (either inherent or applied and a corrosive environment). There are many types of stress SCC. This type of cracking grows over time and the bit you are looking at welding is not highly stressed.
Aluminium needs to be perfectly clean, use a proper welding cleaner, not something like brake cleaner as the residue gives of phosgene gas when heated......not good.
Preheat and maintaining the heat is one of the bigger issues as the thicker sections will suck the heat out and cause cracking.
I think welding as the last resort would be because it would damage the armour hardness, but again thats no big deal unless you need to be bullet proof. I have never delved deep into CVRT armour but often it is the outer surfaces that are hard with the centre normal. If you have an alloy composition I can dig around for a welding procedure