"If you have the current engine number, we can check whether it’s original."
Do you think it's safe to assume that Alfa Centro Documentation is simply confirming if the engine number falls in the range of chassis numbers for a given model?
They have specific documentation that match engine and VIN number.
The answer depends on model and year. I've been to the archive myself and have seen the 2600 documentation (I was granted complete, unsupervised access for research I was doing). These records are handwritten accounting ledgers, not build sheets, and there is NO information in these books concerning engine numbers.
But, it appears that when Alfa switched from handwritten records to computerized records for later models, they seem to have recorded engine numbers as well (for example, compare with what D'Amico & Tabucchi show on p. 13-16 of their book -- the computer printout for 105.12 (2000 Berlina) cars from 1971 seem to have chassis as well as engine numbers listed for each car, as shown in the picture below). So, yes, for a 73 GTV, I would expect that they have computerized records with the actual engine number, not just a number range.
However, there may be corporate policies in place that prevent the archive from releasing engine information up front where such information exists. They may be allowed to tell you if the number matches, not what it's supposed to look like -- similar to a testing whether your key fits the lock rather than telling you what the key for the lock should look like (e.g. to prevent fakes, etc.). By the same token, it appears the archive can no longer release the name and/or city of the first owner, even if they know it (which would only apply to sales in Italy).