Sounds like a ton of fun. I have not hunted there, but I would think the areas above and to the south of cow creek would be great. Above timberline in the difficulty creek and wildhorse creek drainages. I'm guessing that is more like 4 or 5 miles from the end of the road though. Might be tough to get one out if you don't have mules.
I think that 64 is mostly too low for Elk in September. But I have not spent any time in 64 except in the NP.
Muzzleloader season is 9/14-9/22 this year.