240 body is known for rust being a common issue in salty areas. However there are things you can do to reduce the tendency. Beyond that, only option is to live with it.
If there are any chips or breaks in the paint, seal them up immediately to keep rust from being able to take hold.
Oil coating on underside is sometimes helpful (many folks in the US Midwest swear by this).
Any rust repairs, make sure they are sealed up well with good paint or you'll see it come back.
It's a never-ending battle! Part of the "fun" of owning an old car. But if you can keep it stabilized, it can be managed.