The Shaffer furler that is installed on the C-320 Mark II (and for that matter, on must of the Mark Is too) is a roller reefer, so you can use it to reef down the headsail, not just furl it (unlike the Hood continuous line furlers on earlier boats (such as my two boats ago '87 C-30T)). Therefore, a low stretch (not the lowest stretch possible, but reasonably low stretch such as Samson XLS Extra) furler line can be a slight improvement over a medium stretch line such as StaSet.
Removing the first 20' of core (as measured from the drum) makes the line fit the drum better without over-rides and jams, and you would want the 3/8" for easier "hand".
Steve
Alchemist C-30T #4764
Alchemist C-320 #909
Alchemist C-380T #71