I have and still do use the N-9y plugs (when I can find them) in my 69-70, 340's. The "R" are for the radio interference and the "C" is for copper. I have heard that the resister "R" plugs, fail often. I run copper spark plug wires and copper dist. caps , so why not use "C" (copper) plugs. They work well for me. The last plug change I did had over 60,000 miles on them. Setting the gaps correctly are important, as well as the points. Always install a new condenser when changing the points, it prolongs the points.