404's for the 1600 too. The 64 is the same thread/heat range/ thread
reach, BUT the 64 is an extended tip plug Take a look at the two together at
the parts store and you'll see what I mean. Your roadster will want the 404.
I'm surprised the catalogs are showing it that way. I dug out a fairly
current autolite catalog (1998) and BOY it is sketchy. They may be referring
to the L-16. I also checked a handy NGK & Champion catalog and they dont even
list the 311's. (NGK equiv of 404 is a BR6ES btw)