Phillip,
I built one of these kits a few years ago. I'm not at home right now, and can't properly "credit" this, but maybe someone else on the forum can. I have a book, titled like "The American Frontier". Believe it's edited by Wm. C. Davis, but that could be off. Anyway, there are lots of photographs, mostly of items in the Buffalo Bill Historical Center, and there is a layout on one of those pages that includes two Hawken Rifles, one iron mounted, the other brass. The iron-mounted rifle is very, very close to a GPR, and would probably suggest to you a few modifications. As soon as I'm back near that book--probably tomorrow--I will see if I can get a decent scan of that pic and try to send it to you. My GPR's .54 cal as well, and I truly enjoy it. If you're not aware, it should have two rear sights, a modern adjustable, and a more pc fixed in the kit.
Take care, and I will send as soon as I can.
Hugh