In 1976, I had a similar problem, the failure of the threaded subframe steering box mounts on my '61 Ponton 180b. My factory Ponton shop manual stated that if there was any doubt as to being able to reliably repair the subfame, it should be replaced.
Yours looks like a fatigue failure, not rust.
The crossmember/subframe is detachable, not part of the unibody so, worst case scenario, you'll have to replace it with a good used or new one.

Happy Motoring, Mark

__________________
DrDKW