I have used the OEM plug heat range, N9YC non resitor or the resitor # RN9YC Champions in many 1968 to 1971 stock type 340 with great success I would use the better RC9YC now, they are the 5/8 socket plug size, a little easier to install That or the hotter RC12YC if your carbs. are rich at idle and part throttle In Autolite I would use either the #64 or #65 or the resistor plug # 84 or 85 Both brands should work well and there both easily found at most auto parts stores