At this point you may be wondering WHY NOT platinums.
There have been a mention or two of rumored catastrophic failure involving a platinum plug breaking and ruining an engine. I would think that the chance of this actually occurring would be remote.
The practical reason we use copper core plugs in the six cylinders is that the platinum plug has a very thin center electrode for cost saving reasons. This thin electrode produces a thin spark. The copper plug with its wide electrode produces a fat spark. In the sixes of this vintage, and I expect the V8's as well, the ignition system will not produce enough energy to give a smooth idle with the platinums.
Just thought you might like to know.
Have a great day,