Used to get em running after they sat in fields for years. you are correct youll know right away if it needs torn down. I wouldt get all surgical because its a 340,id get it running and see what it does if it makes noise and smokes Id tear it down. minimal rust on the bores will polish off if it is too bad the rings will break and youll know. If the rings dont seal youll know rather quickly. On the other hand if you want to open it up and you have the $$$ and the skills go for it. these cars are easy to wrench on.