Another leaf spring question

I would definitely do a thorough check on the rest of the car and chassis, how much undercoating you remove is up to you. That kind of rust is usually not isolated to one spot, and if it’s that bad in one spot then there’s a really good chance that it’s that bad or worse at other locations too. With rust there’s always more than what you see at first.

As RRR said, it’s just about impossible to repair sections at locations where there’s rust like that between the layers, you just can’t weld to it without pulling in all the oxidation. Obviously I’m not looking at it in person, but I would not be surprised if repairing that mount includes sectioning both the frame rail and rocker, at the least. That corner with the mount is a complex joint with overlaps of a lot of different pieces, and it’s a major structural component for the chassis because it ties the frame rail, rocker, spring mount, and floor together and serves as the base for the quarter structure and C pillar.