1970 340 SPARK PLUG RECOMMENDATIONS

In 1984 I had a 69 340 swinger for a work car . I bought a 318 car to part out and there was a set of new N12y's in the box on the seat. I thought great a free tune up. I came home from work and put the used N9y's back in the next day and tossed the 12's. Probably would be different with todays fuel. I would say install a set and read them.

in the 80's I would drive the car get it hot. pull over on a long straight and install one plug. Have one plug of various heat ranges with you. Start the car and do a 1/4 mile run. shut the car off coast to the side pull the plug and read the strap. If its burned back past the bend go colder and try again. Every motor likes a different plug depending on fuel and motor build. Most 340's did like N9's or 63's. These new High energy ignitions with multiple spark and todays fuel changed most needed heat ranges for many motors. You cannot go by those old books anymore except to get close to what you need.

You could take it to a Dyno shop and pay them to do it for you. $$$