That "blind" area is a prime candidate for practicing patch panel welding, imo, but I've left/welded over far worse. If the rust hole is staying, I'd just prime it & weld the replacement pocket over wet primer- after using some sort of acid wash & thoroughly neutralizing it. The replacement piece almost looks like a perfect fit. Are you cutting out a square or rectangle spot for it to be welded in?
For longevity of 75 years, or so, I like 2 part epoxy primer. I don't know which rust converters are "ok" for epoxy primer because I've only used the 2 part primer on new and/or media-blasted metal.
Brad
