G-body rust repair

[BigBlockOlds]

Any ideas on how to go about repairing rust in the location of the body mount area at the front edge of the rear seats?

Here's a few pictures of what I'm dealing with:







This entire crossmember piece is probably not reproduced by the aftermarket is it? Seems the best way to tackle this would be to just weld in a good crossmember/body mount piece but finding a nice one of those in a junkyard will be next to impossible.

Thanks,