The lower section behind the driver had to be cut out. That's the skin on both sides and the main inverted U channel across the bottom. It was beyond repair and so I cut back to the good metal. A new beefier rectangular box section was prepared with the bottom edge cut away near the outer edge of the bus to permit access to the reverse side (enabling connecting the new floor channel bracket for bay one). This was welded to the front bulkhead and given the 'hit the darn thing with the biggest hammer I have' test, which it passed (but hurt my hand). A section of steel was cut for the front skin and welded to the front edge of the bulkhead. All the surface and minor rust on the rest of the bulkhead section was treated with rust treatment. That had to be left for a day to do its thing. After which the section inside the bulkhead area was painted with black bitumen paint. It may never be seen again, but I don't want it to rust from the inside. The picture below shows this stage.
This needed another day to dry, but I gave it two days to be sure.
Then today the rearmost face (already treated) was welded on. Some seam welding and some spot welding (as appropriate) and also prepared a lower section step to reproduce the original profile. Once welded on and cleaned up, the whole section was painted with black bitumen paint. The final outcome is pictured below.
I can move on to the next bit now.