As far as repro Hawken rifles go, the difference in barrel length will depend upon whether you are using a parallel or tapered barrel. You can get away with a lot more length in a tapered barrel than one that is parallel.
For a straight parallel barrel (of coarse it was straight!) the very nicest shouldering Hawken I ever saw has a 28" x .54 cal x 1 1/8" barrel, and the stock has a 13 1/2" pull. I'm 6' 2" tall with long arms, but this rifle really felt wonderful.
I have a friend in Ft. St. John BC who has a .54 x 36" tapered barrel, and that rifle is a killer (pun intended) too. That rifle has a 14" pull I think. He's hard to beat with that rifle.
So, I'd build what you can comfortably shoot and carry. A heavy rifle wears you down shooting offhand over a fifty target trail, but for those first difficult shots, the extra muzzle weight sure gives a steady hold.