Hi Turniphead
I used to position singles in the United States for trans Atlantic ferry, and and went across in a few twins with that pilot. Thus I have a little experience over the ocean. With this little experience, and a lot of experience flying 172's, my advice tends toward the dire side. Hire a container, put it on a ship.
It can be done, but just. The 172 does not have the capacity to allow comfortable range and choice in alternates. Suitable landing areas across the Atlantic are too few and far between for lonely 172's. It also takes a lot of emergency equipment and knowledge to satisfy Transport Canada (who regulate these flights). Look here: (602.39)
http://www.tc.gc.ca/CivilAviation/Re...602.htm#602_39
You will find pilots willing to undertake this flight, but that is not an assurance that it is an ideal way to get the plane across the pond.
There is guidance material available for ferry tank installations if the pilot is insistant on this kind of flight.
Think it over carefully,
Cheers, Pilot DAR