Spark plug selection

the only difference between the champions c series plug and the n series , is the socket size required to install them. so, yes you could use a rc9yc plug instead of a rn9yc. i have used the champion rn9yc plugs in my 340`s since the 1980`s. i even still used them in my 360`s with cast iron heads. when i switched to edelbrock heads, i used the recommended rn12yc plugs as per edelbrock instructions. the 340`s from 1968-71 came from the factory with rn9yc champion plugs. if they were good back-then, they should still be. i understand that this was for street use.