Are you 100% sure if the bottom bracket threading is Italian? Because it could very well be English/ISO even if it's an Italian frame. If it is indeed Italian, Omniums will not fit on your frame because they don't make Italian bottom brackets for the Omniums. On the plus side, Shimano makes several bottom brackets with Italian threaded cups; as well as Miche. Ask to see if your local shop could get you any of those.

Because a set of cranks determinate what sort of bottom bracket you need, if you buy the bottom bracket first, you're kind of painting yourself into a corner if you know what I mean. Bottom brackets have different lengths, tapers, lengths, some are asymmetrical, some aren't. By deciding what kind of crankset you're getting...let say a Sugino RD-2, then you could buy a Shimano bottom bracket with Italian threaded cups.

I doubt your De Rosa is a counterfeit by the looks of the welding of your bottom bracket shell junction. You've got a great frame!