I've flown through the Fortymile region a bunch of times and I've even managed to see a caribou or two. In mid/late August they are most likely going to be near or above treeline on high hills, plateaus or ridges. Think lots of rocks, scattered clumps of berries or brush, few scrubby spruce...maybe some tussocks. Visibility shouldn't be a problem. As August gets long, the migration begins toward the south and east generally. I've had years where hundreds of them came through my area in just a few days. By September 10 many of them (if not all) are out of the alpine and long gone. Mid/late August should be prime for the GMU you're in, and then it comes down to your pilot-friend being able to place you near animals. If that doesn't happen you'll see few or no caribou...the Fortymile and Nelchina herds can be notoriously unpredictable and hard to locate. That's the nature of caribou anyway; they're evolved to be wanderers and not homebodies.
I'll be in the Fortymile area in late August myself...fires permitting. I'm going up alone and cubbing in to an area I've never hunted before. I'm hunting solo for 8-10 days and I expect to see plenty of caribou...but not every day. I'm looking forward to the challenge and adventure. Should be a good one.