Hello All, I have a 1968 340 in my Valiant and just installed a paxton supercharger

Edelbrock is suggesting Champion RC12YC for a plug in those heads. What you have is the NGK equivelant of the Champion plug.

Below is a chart of NGK heat ranges. The larger the number (i.e. 6 is colder than 5 etc). I believe Champion is opposite of that, smaller number is colder.

If it were me, I'll take a NGK copper core over a Champion and try a heat range of 6. My 360 magnum calls for the same plug as yours and I had pinging issues with it. I changed to a colder NGK plug (6) and solved the problem. Every application is different, you're going to have to learn how to read the plugs to know if you have the correct heat range if you don't already know how. Hope this helps.

ngk spark plugs heat range - Bing images