It might sound obvious but do make sure that you have a visa. Ferry flights do not count for the USA visa waiver programme.
Make absolutly certain that you have enough fuel to divert from over head Narsarsuaq. The weather can change very quickly there and as its only an NDB/DME approach the minimums are high. (yes I know you will be doing it on the GPS but!!)
Make sure that you have a pee bag (can buy them from transair) even if the aircraft has a pee tube it may not have been used for a long time and might be blocked.
Battery heated socks and hand warmers always worked for me (a gliding club may be able to tell you where to get some) If the heater packs up you will get very cold, very quickly. The cold at -30 starts effecting your mental descision making as well. Which is why two crew are safer in that situation.
Even if the other pilot is experienced, always feel free to question him. I did over eighty transatlantics in various general aviation aircraft (C172 to B200) and sometimes its easy to lose sight of the vitally important things because you're in a C421. Pressurised, de-iced etc.
Do think about what would happen if the systems stopped working. If the aircraft has come from a humid environment moisture may be in the pipes for the de-icing system. At temps below freezing it can freeze and you suddenly have no deice. Ensure you test it at altitude as soon as you can.
Personally I would add Ispropoline (spellings probably wrong)Alcohol to the fuel. (Can get that at Wick airport) It can prevent injector icing. Not very common but gives you a real good wake up call when both engines start to run roughly.
Finally do NOT think about buying a book by Ed Carlson "flying the Atlantic safely" I personally feel that it has no useful value whatsoever and at times borders on the dangerous.
Enjoy your trip