The conventional wisdom on coolant appears to be to use inorganic (IAT) antifreeze over organic (OAT) for classic cars. IATs tend to be blue, OATs tend to be orange. IAT doesn't retain its effectiveness as long as OAT, but it is better at preventing corrosion within the system. That's fine if you change it every couple of years, which most classic owners would.
That said, I've been advised against mixing the two substances, so you should top up with whatever your existing coolant is unless the system has been drained and backflushed.
Hope that helps.