Before I laid the new floor, I spent some time cleaning up the frame underneath and treating the areas of surface rust I found there.

I also had a mate of mine come out with his welder and weld in some extra steel beams into the existing frame – mostly across the aisle, and also across the large spans on the sides. This gave me a much better framework to support the smaller sheets of plywood I would be laying down.
I measured and cut out all the sheets of plywood and sealed all the cut edges with a waterproofing bitumen paint.
I also took the time to create a detailed map of the underfloor frame while it was fully exposed, because I knew it would be a tremendous help later on when it was time to screw down the new floor.