RE: Hawken unfinished kit
If the tang is off center and not level in the stockthe barrel can be off center and could effect your accuracy. It is not all that hard to do. A dremel tool needs to be used with caution because there is only a small amount in most cases that you need to take off to make the parts fit flush. When I did mine, I used a scraper and sandpaper.
As for browning or blueing the barrel there are a number of different products on the market you can buy to do that. Some of the better ones are made by Brownell and Birchwood Casey (only my opinion). I browned mine with some stuff 25 years ago that I do not even remember the name of anymore.
Yes the barrel is already rifled. It comes with a 1-48 twist. And the breech plug is installed in the new ones I think. So once all the parts are set and finished the gun will go together and your off to the range. Another kit you might want to look at are the Lyman Great Plains Rifle and Great Plain Hunter. I would get the .54 caliber but that is personal preference on my part.