Now that I've got the chassis straight (see 'Puzzling body twist' thread), I'm beginning rust repair. I've removed the outer clamshell on the passenger long. This thing was rusty more than 33 years ago (because these repairs weren't done during my ownership) and someone had scabbed on patches to hide the rust and add strength. I think the front floor is salvageable and so I carefully removed the scabbed on patches that were on the inside of the cabin and underneath the car where they attached to the floor and long, also where the jacking donut was.

The outlined area in the pic below shows where the metal needs a patch. The flange where the floor gets plug welded to the long can't be perforated with a pic, but it's not looking very good (the silver metal in the pic is remnants of the patch that was scabbed onto it).

Click to view attachment

If I cut a piece of 18 gauge per the lines I've drawn and butt weld it, along with the long repair work etc. I will be doing, will that provide sufficient strength in the area of the jacking donut? Or, do I need to add a second layer somehow to strengthen this patch?

A butt weld ground and blended will be just fine for repairing this area..

Great, thank you very much for your response!