In the Italian Wiki I read that the entire city of Ravenna is included in the park Delta del Po which belongs to the inscription of Ferrara and the Po Delta. Can anyone confirm this information?

Ok - now I can see it on the official site of the park. It specifically mentions Ravenna as well as other historic cities as being within the park boundaries. The park in its entirety is designated as a WHS (mind the WH logo on the right).
http://www.parks.it/parco.delta.po.er/Eindex.html

The official Ravenna site doesn't state that explicitly:
http://www.turismo.ravenna.it/contenuti/index.php?t=areanat&__utma=208035351.96967961 7.1252102404.1252102404.1252102404.1&__utmb=208035351.8.10.1252102404&__utmc=20803535 1&__utmz=208035351.1252102404.1.1.utmcsr=google|utmccn=(organic)|utmcmd=organic|utmct r=turismo ravenna&lang=2

I don't think that the "park in its entirety" is inscribed as WHS (Though there seems no doubt that the whole of both Ferrara and Ravenna are within the boundaries of the Parca Delta del Po. We noted earlier that Pienza is totally within the Val d'Orcia park - Italy seems to like including towns within its "Parks" in a way which many other countries do not. The Parco d'Orcia is even called a "parco artisitico naturale culturale)

a. The Site map for Ferrara dated 2008 in the UNESCO documentation
http://whc.unesco.org/download.cfm?id_document=101252
shows the core and buffer areas in green/pink. The Po Valley areas beyond Ferrara itself are quite "complicated" and spread out following (some) individual "streams" of the delta but go nowhere near Ravenna - and no further south than near Portomaggiore and theenvirons of the town of Commacchio as far as I can make out (The map zooms quite well). The Valli di Comacchio" which comes fairly close to Ravenna is only part of the WHS buffer zone.o

b. the extension beyond Ferrara itself for the Po Delta made in in 1999 is described as "Ferrara and ITS Po Delta" with emphasis on the "Este Ducal Residences". This implies that only those parts of the Po delta considered to be connected closely with Ferrara were added (The additional criteria added with the extension were both "cultural")

Other documentation is "thin"! The ICOMOS review is shown as being dated 1995 AND 1999 - but as far as I can see there has been no section added or updated for the 1999 extension as is normal. I note also this "decision" from 2008
http://whc.unesco.org/en/decisions/1737
in which Ferrara is present with a number of other Italian sites in a list of those which have had their boundaries clarified as part of the "retrospective inventory". I think we can take it that the 2008 map reflects this "latest thinking" on the boundaries!

The use of the UNESCO logo implying that the whole park is inscribed is typical of a number of other places which obfuscate the UNESCO boundaries seemingly to enable them to claim inclusion! Our earlier discussions re The Loire Valley and the town of Angers were similar!

Now that the boundaries of the Po Delta inscribed within the Ferrara Unesco nomination are clear I would like to make some suggestions for conncections:

the Etruscan necropolis of Spina near Comacchio is within the core zone (connects to both Etruscans and Necropolises), Canale San Nicolo-Medelana (canals), salines of the Valli di Comacchio (salt), Comacchio itself is dubbed Little Venice (la piccola Venezia) (dubbed as another WHS), was destroyed by the Venetians in 854 (destroyed during invasion), then it was part of the Republic of Venice between 1508 and 1598 (Republic of Venice), Valli di Comacchio are the most important centre of orinthology in Italy (animal migration), and offer an extensive irrigation system (irrigation and drainage), Pomposa Abbey within the core zone is a Romanesque Benedictine Cloister (Benedictines and Romanesque), Ospedale del Delta in Marozzo is within the buffer zone (hospitals).