Hello
This is just my opinion, from running old cars and being in an old car club (of the vintage you are talking about). The engine and materials used to produce it are no different from any other around at the time, though no doubt some dealers will try and gold plate their 'special antifreeze Sir', or at least it will seem that way when you see the price. Its just an engine, mostly of cast iron, aluminium copper/brass and rubber in there. Same as any other
So
I would just go down to the local motor factors/accessory shop and buy me some decent quality anti-freeze/summer coolant/inhibitor. I would not get any pre-mixed de-ionised water/demineralised water stuff. The minute you put it in its not de-ionised/demineralised any more as it dissolves sufficient metal to replace the minerals its missing. Not what you want.
I would then mix it to the minimum concentration required. For the UK stuff that's about 30% for about -20C protection (sorry, don't know off the top of my head what that is in F). Tip that in and top/bleed it up and you'll be fine.
Don't be tempted to use neat antifreeze. Its not as good at cooling as water on its own, but we can't use just that. Its to do with surface tension, and how antifreeze increases the surface tension, which makes it not adhere to the metal surfaces inside the engine as well, resulting in gas bubbles forming creating localised hot spots.
As I said, use the minimum mix you need for the degree of frost protection required.
If you want to, you could get a coolant additive such as 'water wetter' or similar (that is the only one I can think of, though tere are several). This counteracts the surface tension increase caused by the anti-freeze (i.e. it reduces the surface tension) making the coolant cool better (they claim almost 10C reduction in temperature).
Just had my 37 year old rotary engine in bits. Lots of aluminium and cast iron. They are famous for dissolving internally if you don't use anti-freeze. Mine looks like new. A good quality anti-freeze is the simple advice.
Cheers... Rob.