The stainless is type 430. Melting point not much different than steel sheet metal. If you plug weld the angles, the weld is not exposed to the weather, so we suggest using typical steel welding wire such as ER70S-6 for mild steel. I don't see a benefit to using stainless wire in this situation. Lay the bedside horizontal and use an aluminum or copper backup block to keep from burning through the angle. The stainless will turn blue at the weld area but unless you really burn it through, it is still stainless steel that is exposed. Use some chrome polish to remove the blue scale.
Mar-Ktech
