Be careful. Champion has NOT said to use that 9 in early 340’s since the early 1990’s. You could be stepping on your pee pee.
There is a reason catalogs get updated.
Let the plug that’s in there tell you what heat range you need and know this for a fact. Those cross reference charts are about junk.
You can take a Champion, an NGK and an Autolite that are supposed to be the same heat range and all three will read differently in the same engine with the same tune up.
So what I’m saying is don’t use an early catalog to pick your plug. Use the latest catalog and then look at the plug and it will tell you if the heat range is correct.
I may at some point post some pictures of the three above mentioned plugs so the difference can be seen.
