Love the Darkside technical vids!
Not sure personally, it's a custom race car build remember so likely lots of custom bits and also modifications to make stuff fit.
I wouldn't worry about the rust on a subframe, it will take a very long time for a hefty piece of metal like that to be weakened by rust, if there is any on there now I'd be seriously shocked if its any more than surface rust, if that concerns you then you could always just clean it up and powder coat it. Even if it does rust through one day, they're not expensive for a good condition 2nd hand one.