Whatever you choose, I would make sure have the old metal and the new piece joined so there is no unbonded edges (outside or internal) or unbonded faying (contacting) surfaces in the case of a flange. Leading or brazing is one way--get metal fume fever and sicker than a dog once from fumes and you won't forget it. I tend now to use Sea-Going Poxy putty on my non-structural flange laps and bury the edges in it and avoid any fluxing problems or heat problems. Flanged pieces without a homogeneous bedding of either bonding metal or "glue" will ghost in the final finish from uneven heat dissipation when set out in the hot sun. The edges if not submerged in metal/glue will be a line not able to give up its heat uniformly to the rest of the panel with a heat flow path. They grow more in length. Take it out of the faint line(s)--disappear. This was a previous topic on this forum. That poster wondered why his partially welded flanged panel showed a line in the paint only when out in the hot sun.