The '66 AC Spark Plug Catalog shows spark plug 45NS as correct for the 305E engine. That crosses to the AC R45XLS, available currently. Both are 3/4" reach plugs. The original 45NS was partially threaded, and the R45XLS is fully threaded on the reach. Shouldn't matter, nor should the fact that it is a resistor plug. If originality is any concern, NOS 45NS plugs pop up on eBay occasionally.
Doug
