After following the installation method mentioned in its review, Noctua's NH-D14 presented a rare but recurring theme. Contact between the CPU cooler’s fan clips and a graphics card could have caused an electrical short if I’d powered-up the platform.
The protrusions are finger hooks that aid in fan installation and removal. Luckily, you only need to unhook one side of the fan in order to slide it over for removal. I used a pair of pliers to flatten this part of the clip. But be careful. Unlike mild-steel wire, the fan retainer appears to be tempered, and will likely break if bent too far.
While the top card can still be pushed over far enough to touch the cooler, that probably won’t happen when everything gets screwed down. Besides, it probably wouldn’t be a problem if the parts did touch, since the closest point is now between the GPU cooler support bracket and CPU cooler fins. That possible contact point is grounded on both sides.
Everything fits as it should, and we even have enough room to add a second front fan if needed. Since the PC-9N isn’t designed to hide cables, I turned the removable drive cage sideways and looped the extra length there.