The 1968 to 1971 340 motors came with N9Y plugs originally
![work work](/ubbthreads/images/graemlins/custom/work.gif)
They are not available any more I've been told so the closest one to them is the RN9YC resistor plug and then the resistor 5/8 body RC9YC in the Champion line.
I had a 8.5 to 1 compression 350 Chevy iron head motor in a 21 ft.inboard out board ocean boat that would ping with the stock RJ12Y plugs in it and wouldn't ping with a R11Y plug in it
![work work](/ubbthreads/images/graemlins/custom/work.gif)
H
![twocents twocents](/ubbthreads/images/graemlins/custom/twocents.gif)
e is at 10.+ to 1 compression ratio so I'm thinking a #10 or #9 champion should be just about right at his altitude