i think the main concern with converting a fulltime 4x4 type vehicle was the possibility of messing up how the car drives, so something that is designed to drive in 2 or 4 wheel drive wouldnt be an issue i would think
theres only a few things that need to be TIG welded and crack tested, parts considered "critical function components" ie brake caliper brackets, front suspension arms etc
chassis/engine mounts/body etc can all be mig welded. you dont have to be a ticketed welder, if you can weld properly youll be fine. the rule book says welds must be done by a ticketed welder OR someone who has demonstrated to the certifier they can weld. i have seen a few ticketed welders do shocking birdpoo welds on cars because they are used to glueing 6mm plate together all day and are quite good at blowing holes in 1mm sheetmetal