The .58 caliber I had built has the 36 inch Colerain Barrel on it. Mine is an Early Virginia model in flintlock. But that rifle is a good shooter. That should be a very nice rifle when its finished. And you're right, that big .570 diameter ball is a beast down range.
Now with the .58 caliber Early Virginia, and the .54 Early Lancaster rifles, I have considered a .50 caliber Southern Rifle having it built. You will like that big .58 caliber. And living where you do and what you hunt, I can sure understand the need for it. And yes, you might need shooting sticks. I know I do, but its because I wore out my shoulders. Tore all the muscles too many times, and just abused them in my younger days. I am sure paying for it now.
Good luck with your rifle Pete. Its nice looking and can't wait to see it in the flesh!