I agree with the Champions and I'll go further... Forget all about platinum, iridium, unobtanium, etc. and stick to good old copper plugs. They may be effective with newer designs, but even with a HEI replacement ignition system, all the 258 needs is a copper plug. The others will cost more and do no better; save your money on this one!
Be kind. If for no other reason, it will confuse the daylights out of people and make them wonder what you're up to. And ALWAYS drink upstream of the herd!