Advice on cage after sitting

Only the link to your "gallery" has shown up. Probably because you were under the minimum post count for posting pics / links.

At that link, the plinth box looks great. Only concern/constructive criticism I would have is that you shouldn't grind the welds off at the box edges/corners in the future. Leave them and leave that much material there, vs removing material. I'm not qualified to pass judgement: just pointing out what someone else who does have authority may bring up.

I recommend you use a wire wheel on an angle grinder or on on a drill (with a face shield) to clean the rust off and to allow you to weld onto that metal again, if you need to continue with the cage onto rusted parts. Alternatively, a flap disc is pretty good at just taking off the surface corrosion and leaving base material. But experiment for yourself.