On welding most anything, I like to run as hot as I possibly can. On my Miller, on anything that isn't Sheetmetal, I run the heat all the way up to the max. Then I adjust the wire speed to match my desired gun travel speed. In my opinion, I would remove the paint around the existing weld and burn in a hot weld to tie in the existing weld and the plate and base metal. That will save your pride. A plate to tie in the outside of the C to form a box will help the strength a great deal, along with another fish late on the outside.