frame rot...

These cars are inherently weak to start with. I'm more than skeptical about those "caps". Caps would be better than what you have. Your floor pans must be gone too, unless they have been patched. Unitized construction really depend on plug welded flanges for strength. Those caps will not be as strong as a complete frame rail replacement or a properly spliced rail with sleeves. I can't find a reproduced complete rail. That would be the best scenario. That and good welding skills. You may need to find a donor piece.