Thanks, LR. My wife asked me why I wasn't upset with the latest discovery, and I told her that it was garage time. There's no such thing as a bad garage day. Setbacks just come with the territory.
Now I need to decide if I want to just look at doing a Safe-T-Cap repair over the damaged area of the torque box, or actually consider replacing both torque boxes completely. The latter would be a lot more work, what with all the spot welds needing to be drilled out, and such. Plus, the front of the torque boxes appear to be in good shape at initial glance, though I would definitely need to confirm that. Anyway, it's becoming something of a journey of discovery with this thing. As disappointed as I guess I should be about the rust, I'm still so grateful that the frame seems to be solid.