Rust Repair in Difficult Areas

I'm in the process of addressing rust holes in my 1982 280E. I have replacement metal for a rust hole in the boot floor, however I have a hole in the firewall, just under the battery tray (see photos) that I'm not sure how to tackle, as not only is access poor but a lot would need to come out before welding is possible.

Does anyone have experience with rust repairs in this area? What was your approach?