When working with the protonated form (-HCl) of this insidious poison, the best thing to do is freebase the amine and alkylate the deprotonated phenylacetonitrile in one step before worrying about adding a tertiary alkyl group.
I assume you are trying to make a pethidine congener-- I can't imagine what else you'd want this stuff for besides mustard gas. If one uses a large excess of POWDERED NaOH in DMF to deprotonate the phenylacetonitrile, the red dianion will be visible in no time at room temp. At that point add the amine salt, it will be freebased simultaneously as the nitrile is alkylated. If the solvent wasn't overloaded with NaOH, the NaCl metathesis product can easily be detected as it precipitates.
One final word: Straight demerol can do some really nasty things to your heart. If you must make this krappy drug, alkylate the amine with phenethyl bromide- the potency is doubled to trippled and it is not nearly as bad on your cardiac function.