Quick Answer:
A 50/50 mixture of conventional green (ethylene glycol-based) antifreeze and distilled water.
Long Answer:
Care must be taken to avoid use of any modern orange (organic acid technology) antifreeze in a classic car as the additives will consume the lead solder in the radiator and damage the cast iron water pump body.
Similarly, you should avoid a mixture of the two since even a small amount of the modern solution will damage the cooling system irreparably.