I would suggest putting a straight pipe section before the reducer. So shell -> pipe -> reducer -> weld neck flange (or straight pipe and slip on). It makes things easier for the welding. I am not 100% positive that you can weld a reducer straight to the shell, but sntman didn't bring it up so I am assuming code doesn't have an issue. Even so, I am assuming the nozzle is quite small, since it is exempt from UG-36 reinforcement calcs, so the extra cost for a small straight section should be minimal.