63 Jalopy,
Get yourself a small piece of wire or welding rod, then insert it into the groove around the central electrode in the spark plug. The deeper is the hotter one. You can start with one set, if You for example pick the cooler set and then experience some fouling up, You can swap to the hotter set, or the other way around, start with the hotter one, if detonation or dieseling when shutting off the engine occurs, swap the cooler set. No definetive rules apply, unfortunately I do not have a cross reference between these plugs You have and some others I use here, and also as long as our gasolines are not the very same thing, it'd be also of li'l help.
