Tap water is typically loaded with dissolved solids that will boost conductivity. As a result, you can create a galvanic corrosion cell within the cooling system.
The stainless freeze plugs referenced in here (US Tool) have been great for us. Stainless has a lot more strength and is stiffer than brass so they tend to stay in place better as well.
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'68 H/O W45
'70 Cuda
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