Fortunately, there's enough left of mine that it's still solid. Still, I've had some rust damage on the piece you describe.
Now for the good news.
The crossmember/sill/body piece (whatever it's "official" name is) has now become available as a replacement part in the last couple of years. Try looking around, but I do know that Raybuck in Punxsatawney, PA offers them (
www.raybuck.com ). It seems to me that I've seen 'em a couple of other places (maybe LMC Truck?).
I was exploring that route myself when I was poking holes in that piece on my truck a couple of years ago and farming out the repair to a muscle car restoration guy. The Raybuck guy told me to hang on and that the part was coming soon. In any event, I was lucky that there's enough left to still be structurally sound and didn't end up needing it. A year or two later, I now see that piece available.
Good luck!
Kevin