Almost every rib I have removed showed rust between the bbls. Par for the course. You will never have peace of mind until you remove the rib, clean it up, and put it back on. (Now that you have oiled it, you can't spot repair it...and since it was rusty, you probably couldn't anyway.)
Regarding the cost and time, if it's done carefully using paste solder, you can avoid a reblue. And it doesn't take 12 hours, unless your smith is a union man.
I think some Spanish guns have brazed ribs...better check before you start because that's a different story.
