You can't have too many GPSs & you can't have too much fuel...
You are supposed to have a HF radio, but most light aircraft don't. You prob will not be allowed into Shanwick airspace without one so will be restricted to flying below their level. Without a HF you can pass position reports via satfone (you can hire one if necessary) if relay through an airliner does not work. (All airliners listen out on 121.5 and it is often used as a chat freq across the pond).
Use good survival gear & a 406 PLB with GPS.
If you have not already got one, have an engine analyser fitted (EGT and CHT on each cylinder); cut a filter and have the oil analysed before you go to minimise chances of an engine event.
Don't go into Narsarsuaq unless the weather is good.
Try and get hold of an article in Pilot mag written by Sir Torquil Norman about an East - West trip he did in a Fox Moth; one of the best I've ever read... Watch out for icebergs at low level
In case you do not already have it you will need this:
http://www.nat-pco.org/nat/CurrentNAT%20IGA.pdf
Don't skimp on the fuel minimums mentioned in there - You'll need to fit some sort of aux fuel tank in your RG - make sure it all works as advertised before you need it to..
