I think I solved the problem. This is my first OC card - I didn't realize they can come overclocked from the manufacturer. My card was running at over 2500 MHz - well above any recommended boost clock settings - which is why I was seeing such high temperatures. I lowered the GPU Boost clock to 1826 MHz in GPU Tweak and can now run Warhammer III on Ultra without crashing -- the GPU junction temperature never went above 54C (I also adjusted the fan speeds as Cyberat suggested). The CPU temperature still crested at 76C but that isn't causing any issues and I haven't changed the fan settings in BIOS yet.
Going to fiddle with the boost clock and see if I can squeeze a little more power out of the card but for now things are working smoothly. Thank you all for your help!