I respectfully disagree that a flint Hawken is as easy as any other longrifle. The hooked breech, long tang, long double set trigger plate, wedges instead of pins, and inletting a deeply curved, steel buttplate are all more advanced tasks. I love a Hawken, I purely do, but I only ever built three, and won't do it again unless I get really inspired.
It is rare indeed that the trigger plate is properly curved, and when you bend the plate, there is the possibility that the triggers don't engage properly anymore. If you think about it, the length of pull and etc will affect the angle of the underside of the buttstock, assuming you use the same length buttplate.
That's just one of the challenges.
I always advise building a trade gun or musket for the first build, or better yet, putting together a kit like the GPR etc.