Mostly stock 340 engine what Spark plugs

RN9YC was the recommended stock plug for 340 '68-69 and I presume '70 and '71.
That's a slightly colder plug than the RN12YC. Without testing, its hard to guess which one is best for your motor today. If you go by the the theory "run the coldest plug that doesn't foul" then the slightly colder plug might be the one you end up with.

If you don't like Champions, just find a cross reference chart from the manufacturer you prefer. IMO Autolites are harder to read.