Your Levis probably uses a 14mm spark plug, being a late model.
Here are details of some plugs: -
Lodge H1 (18mm) or H14 (14mm)
Bosch DM175T1 (18mm) or W175T1 (14mm)
Ofcourse these are the original plug numbers from that period, so you will have to find the modern equivalents.
I suggest you contact Tim Green at The Green Spark Co.
http://www.gsparkplug.com/