I amputated the ribs, then reknit them upwards with fewer sitiches.
I don't know how I cast on using a simple long-tail cast on and it's perfect, but even a loose bind off is too tight.
I cast off using Jeny's surprisingly stretchy bind off.
It's a bit flary, but this seems normal for the technique, and it functions as a stretchy bind off, so I can let it go.
I'm on the decrease section of the Born Trippy ... pics tomorrow!!