Do what I suggested years ago when I found some like that, only the ones I found were half full of water - pump them full of concrete and install new ducts overhead.
Of the house I did, in Palm Beach (yeah, it was expensive, but it had NOT been maintained, you would think that people with money would also have common sense ... wait, why on earth would I think that

), and was two story, so it *was* a problem, but I saw no other solution. Not that big of a problem in the realm of things though, the house needed a LOT of remodeling and deferred (being nice) maintenance, so install the new ducts then.