Hi, I am a professional certified mechanic, with an engineering background, and 35+ years experience. I will do my best to assist you. Also keep in mind I don't know if you are a pro or a novice, so feel free to add any additional info at any time.
The rust can cause a lot of problems with heat transfer, and trigger overheating. On a small block chevy the water pump impeller is thick stamped sheet metal, it can rust away where the pump isn’t moving enough water. It is real tough to tell if this is the problem without pulling the pump off, and taking it apart. If you take it off a rebuilt or even a new pump is pretty cheap, ($20 to $30) and it would probably be smart to just replace it while it was off. You might also want to flush it out with some type of strong flush type product. If the radiator is plugged up flushing can help if it is the powdery rust residue stopping up the radiator. A little antifreeze will help prevent more rust from forming, but antifreeze wont clean up any rust you already have. If you have any more questions at all, I am here to help. Good luck with it, have a great day, and Thanks for using Just Answer.
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