On mine the cat flanges fixed to the manifold had hardly any rust on at all.
The lower cat flanges connecting to the Y pipe were rusted away to less tha half the original thickness and worse in some places, as were the pipe flanges attached.
The USA fix would be well worth it even using mild steel again, mine did last 17 years!
Proves it could be done, alignment would be needed before final fix though,
flanges are angled.
Saving megabucks!