it's great to hear some of my German compats are planning to come across the great pond to tour next summer. That said, I have not seen an ACA map that would lend itself to serve as a blueprint for your Boston-Montreal-NYC tour. However, parts of the Atlantic route combined with some that others have taken may provide for good options and opportunities to hear what others have experienced. Check out the tour journals on www.crazyguyonabike.com
, if you haven't already, to see what others have done and read about their experiences. There is a wealth of information in those pages and I'd be surprised if you can't find someone who's done either leg of your planned tour and whom you could also contact for info directly. If you can't fine anyone, local maps can help to plan your trip but they may be difficult to get a hold of from Hamburg and will likely miss much of the information you get from ACA maps such as places where you can charge your gadgets, local bike stores, etc.
Alternatively, if you aren't dead-set on including Montreal, consider taking the Atlantic route north to Maine and then take the Northern tier route a bit west, or just ride west from Boston, to meet up with the Northern Tier route and take that one to, say, Chicago. The Atlantic route South may be also provide good options and would bring you through New York and possibly on to Philadelphia and Washington, DC. It all depends on how much time you have. If you can be flexible regarding your port of entry, other areas of the country offer a multitude of great tours as well that are accessible from international airports and might fit into your timeframe. I'd recommend you check out the inventory of ACA route maps and consider taking parts of those routes. For instance, you can pick up the Lewis and Clark trail on map 2 by flying into Omaha, NE and return from Vancouver which is near the end point of the Lewis and Clark trail on map 7 of that set.
Just some food for thought. No matter which tour you end up doing, be sure to bring good sunglasses.
The rays over here are much stronger than in Hamburg (NYC is the same latitude as Rome, Italy!) and you'll find you'll wear them almost all the time.
Best of luck and have fun the trip!