I turned the chassis over and removed the severely rusted remains of outriggers and their supports to discover that the chassis beneath had suffered quite badly from corrosion, so I bought some 4mm plate from a convenient local scrap merchant (along with some angle iron to repair the outriggers) and cut and fitted it as snugly-fitting reinforcing plate inside the chassis rails.
On the advice of a friend, I won't be welding them in place, but will be bolting them with high tensile bolts through existing holes (the bolts visible here are only temporary). I'll treat them against rust, then fix them in place when I bolt on those things for which the holes were designed. I chain-drilled holes to accomodate the domes of those rivets which I am not removing. But I have removed the rivets which held the outrigger supports and will bolt the replacements in place.