Measure the thickness of a bare bit of steel on the chassis, which has not rusted and has no paint or lumps on it, with a micrometer or vernier caliper. Do the same to the bulkhead and then purchase the correct thickness.
Make your repairs using butt welds i.e. an edge-to-edge repair and then weld - plating implies laying a patch over the base metal, which is a poor way to do it,
Cheers Charlie

