Not steel, but when I used to braise copper pipe where it went into a building, i'd wrap a wet rag around the pipe which stopped any heat getting past it. I can't comment on how the electrons will react to a wet rag, but I feel like if you had one tucked in against the exterior panel and in that wheel arch lip on the right, it would help a lot. I've only ever done MIG on my cars and a single spot weld in one area, then moving to another area to put one does keep the heat down a lot. Just my thoughts anyway, i'm sure others have done very similar jobs to yours. Only mentioning it as the wet rag wrapped around the pipe was a very simple solution but worked perfectly.