Small Block 305 Bottom end clacking (sounds like small diesel engine)

[81GMCSierra1500]

Another question... Engine ran super hot today (30 minute highway drive (110km/h), checked the oil, was well below the "add" mark. immediately went to get more oil and topped up. But even before this, the engine has always had a mechanical clacking noise come from the bottom end of the motor (most audible from directly under the oil pan). Wondering if this is anything to worry about? The motor is otherwise strong, very little smoke, fires right up, even when cold... just wondering 1. can a low oil level cause damage, and 2. is the clacking a serious issue needing dealing with?

Thanks again!

'81