You say WRC. Is this WRC 107 (or the similar WRC 537) or WRC 297? WRC 297 does not address attachments to spherical shells. Your head is more of a spherical connection rather than a connection to a cylinder.
You say evaluation. These WRC documents do not evaluate stress, they provide a means of calculating (estimating?) stress.
Do you wish to model the relative flexibility of this connection? While both of these documents calculate stress, only WRC 297 provides estimates on shell flexibility. If you choose the WRC 297 nozzle flexibility calculation the CAESAR II "flexible nozzle" input requires the orientation of the (supposedly cylindrical) shell. Some would use a different reference vector for the vessel just to get the model to run. But this will improperly apply some of the stiffness terms. I would suggest using the "custom" nozzle definition so that you can correctly define the flexible (and rigid) directions.