Most people probably know it doesn't really matter, that the RN11YC4, or anything equal to them, is fine for a stock D1, so aren't commenting. LOL
These engines are 1960's technology.
Save your overthinking it for something more important.

 

prior to installing the RN11YC4 's , I was using Bosch Double Platinum, so far I see no difference or reason to go to any of the "high end" stuff out there. I have Magnecore
7mm wires and so far so good.

 

Yes, just pick a spark plug and go with it. On a stock engine like this you probably will not even be able to tell a difference. Just go with an inexpensive copper plug such as ngk, champion, etc. If you had a built motor running 10psi of boost you would still go with a copper as I did. Platinum plugs have such a small electrode that the spark can be blown out or the tips damaged under high boost.

 

which ngk plug is the copper one?

 

The only that does not say "platinum" or iridium" on it.

There are only 3 types of spark plugs, cooper, platinum and iridium.
And the prices will reflect as such too...
$
$$
$$$

 

 

standard plug BPR6ES.........SHELF STOCK# 7131
IRIDIUM BPR6EIX........SHELF STOCK #6637
V POWER BPR6EY.........SHELF STOCK# 6427