Given the limited use of my GTX, several years ago when it had iron heads I used to be able to get at least two seasons of use out of a set of Champions and they were rj12yc. After that, I would change them because they were rusty on the outside, not because they weren't firing.

In the 13 or so years I ran that same iron head 440, I never had a problem with the spark plugs that I couldn't blame on fuel delivery or another part of the ignition.

Some applications might have some magic or voodoo in spark plug selection, but I've never seen it. A spark plug either fires properly every time, or it doesn't. It's either the right heat range, or it isn't.