I plugged in the two 6-pin power cables anyway and closed the side panel and fired up the Mac Pro and I waited for the login screen… and waited. No matter! It's only missing a measly 25W. Each 6-pin power cable provides 75W plus the PCI connector bus provides another 75W for a total of 225W. Unscrew the expansion card retainer plate.
I shut down the Mac Pro and proceeded with the hardware installation. With my new macOS Sierra freshly installed, I was able to successfully install the NVIDIA beta drivers without issue.
