Well that's one way of doing it...
For a start the guns on most planes seem to use the best machine gun in that class (e.g. The p-51 mustang uses the browning) however, I don't know why most of the stuff doesn't have NEI recipes, unless everything uses stuff made in the flans mod work tables, they don't have NEI integration, but I would assume on the page for the content pack you are using is a list of all crafting recipes.. Dunno though
Some weapons use the boxes as an ingredient, some use the box as a crafting table, and for some you can take your pick out of the two methods. But even that takes a fair bit of figuring out! NEI just lists everything as coming from "Flan's Mod", so it's not so obvious which item is coming from which pack, or which box should be used to craft it - assuming you even see the boxes in NEI at all!
It wouldn't be so bad if I were installing the packs myself, one at a time. But in a pre-built modpack with lots of content packs installed it's a bit of a mess. To be fair, the one I'm on at the moment isn't FTB, but I figured this'd be the place to ask.
For example, let's say you manage to figure out that the Cobra helicopter comes from the Modern Weapons Pack. Even armed with this information, good luck finding any webpage which states that anywhere!
In any case, by digging through the archives and acquiring names of the weapon boxes, from there it shouldn't be too hard to build whatever I like. Methinks I'm gonna have to build myself some hangars.