You need to fix it properly. It will leak eventually. I would start soaking it down with some penetrating oil. After letting it soak for a bit, take a soft face mallet and tap the water neck to loosen it.
(There is most likely a lot of corrosion built up inside where the bolt passes through the water neck, holding it from coming off.)
Once you get it off you might be able to grab onto the remainder of the bolt with vise grips. I would use a small propane or Mapp gas torch to heat the bolt and area around it. It should help loosen the corrosion bond. Be patient. It may take a few times of heating and soaking it with penetrating oil.
If the bolt happens to break off flush into the manifold, then the bolt will most likely need to be drilled and tapped.
IMO...Do yourself a favor. Don't put a chrome water neck on. They are prone to leaking. Clean up what you have and paint it.
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GEN III 5.7L "LS6" Engine swap
TKO500 5 spd.
3.54 Dana 44