Dear Ketan,
Generaly the nozzle neck is not reducing one but the flange,
So, their is no difference in the applicable clause for the design of this particular nozzle then the one you design normally i.e without reducing flange.
as far as the applicable clauses,
UG 36 tell you which are the applicable cluase,
you can use either UG 37 through UG 45 or Appendix 1-7 or 1-10 as per UG 36.
and as far as you are using standard flange no calculation is required for internal pressure provided flange rating is higher than your design pressure and temperature as well as hydrotest pressure as per ASME B16.5/B16.47.
WRC 107/297 is applicable only when piping loads are applied to the nozzle.
hope this clears your dought.
Regards,
hitesh Pawar,