DCVB transcribes diftong as "diftóŋk (Barc., Val.); dittóɲс (Palma)" and DNV gives [diftóŋk]. However, English Wiktionary gives: (Balearic, Valencian) /difˈtɔŋk/; (Central) /difˈtɔŋ/.
The main questions are:
Me, as a northern valencian (so I speak Tortosí dialect), I pronounce it [diftɔ́ŋk] (or sometimes, [diftɔ́ŋ]). In cultism, people tend to pronounce <e> and <o> as [ɛ] and [ɔ], even though sometimes the recommended pronunciation is with an [e] or [o], it's the case of <diftong>.
Words ending in [ŋk] can be pronounced only as [ŋ], it's done especially in Central Catalan, but also im Nord-occidental and Tortosí (and maybe in others). But in those dialects you can also hear it [ŋk], so I'd recommend you to pronounce it this way always. The only exception is the word <doncs>, which every pronounces like [dɔ́ns] or sometimes [dɔ́ŋs].