The 1968 to 1971 340 motors came with N9Y plugs originally work
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 outboard ocean boat that would ping with the stock RJ12Y plugs in it and wouldn't ping with a R11Y plug in it work
He is at 10.+ to 1 compression ratio so I'm thinking a #10 or #9 champion should be just about right at his altitude twocents

Last edited by Cab_Burge; 04/26/20 09:29 PM.
Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)