Brett,
Using distilled water alone is not a good idea. Proprietary antifreezes use an inhibitor, thought to be a multi-dentate complex - I don't know what is used. You need that inhibitor to prevent the corrosion which is not due to atmosheric oxygen dissolved in the water as some believe. In the '50's the inhibitor was borax or a borate salt but performance was found poor in repressing electrochemical reactions once we started to see exotic alu heads on iron blocks.
An expansion tank is always a good idea, glancing through the classic motoring comics on a recent visit to Asda ( I'm too imporverished to buy them), I saw an ad for some great accesories, one of which was a nice and reasonably priced stainless steel header tank. Try Practical Classics to see if it's in the latest edition.
HTH
Ozy
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