Spark plugs. Why is it so hard?

If you have TF heads they take a 3/4 reach 5/8 hex plug. Most BBM heads took a 5/8 reach 13/16 hex plug. The two are not interchangeable.

Heat range is a different discussion.

If the reach and threads are the same, then they can interchange to the smaller plug. You don’t want the bigger hex, only difference is the socket size between the two.

I run NGK BKR6E / 6962 in my J heads, Eddy RPM heads and 1.88 heads on my LA’s and my 5.9 magnum Ram.