MNPS
If you search the internet for MNPS (Minimum Navigation Performance Specification) you will probably find the first suggestion is the website paris icao int, and you can download the 2013 latest edition.
It covers everything about flying across the North Atlantic with sections about lateral separation, RVSM etc.
As for where the tracks are, this is done each day based on where operators want to go and the winds. Operators can submit a form call a PRM with their origin and destination for crossing the Atlantic and once these are all drawn on a chart a decision is made as to where that days tracks will be.
Often there is a gap in the middle and that will be to avoid jetstreams against the flow. I have been to Shanwick a few times and seen the chart they prepare. I even have a copy of one somewhere.