You are very lucky and I am very envious...my dream hunt is to do about the same thing, but do it for brown bear, large black bear and moose in Alaska.
Good advice, you'd be kind of "jumping the gun" trying to build and learn the flintlock in that short of time. Definately get and use a percussion rifle for this first hunt. You should really shoot a flinter often for about a year before taking it on that "serious" of a hunt.
You may want to keep the percussion after you get your flinter too, as a "rainey day backup" rifle...I'm a flint shooter but I have a 1861 Springfield for that reason. In other words getting a percussion rifle now won't be an "extra" expense, and if so you can always sell it. But again I find it's a good thing to have a back-up rifle for many reasons.
All things considered and on general principals I would get a .54" for sure if you got a GPR. A .58" would be good if you came apon one you liked, but you'll find more rifles in .54 and it's a very good caliber for larger game, superior to the .50" which is so popular. At any rate .54 would be pretty ideal for large black bear.
Personally I prefer the Civil War rifled muskets such as the '61, '63, Enfield, etc., but for many people these are kind of long and heavy rifles, and not for everyone...don't bother me none though and I love hunting with mine.
Sounds like quite the hunt...wish I could go with!!
Good luck.
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