Rust is quite often found beneath the ribs. Could be because the bbls were browned or blued and some acid and water got in there through a sight bead hole or a loose spot; or because the gun was used in the rain with loose ribs; or because some genius used acid flux solder in the past without washing it before final assy.
The only time I would do a quickie is if one end of the rib loosened up due to mechanical impact and it appeared to be a clean separation. In any case, it's hard to work solder into the crack without using acid flux, which in turn will just cause problems in the future. And if you get it in there, it often sticks to the bbls requiring a rebrown/blue anyway. Rather remove the whole thing and do the job properly the first time. If you want to save money, then do rib removal, clean up, and barrel prep yourself.
However, if the gun is a $500 junker, you can't afford to spend any money on it so either shoot it as is, or cut some corners (as long as you don't create a time bomb).
