[CentOS] Semi-OT: hardware: NVidia proprietary driver, C7.4

This is really frustrating. I've got a server with two K20c Tesla cards. I
need to use the proprietary drivers to use the CUDA toolkit. Btw, I had no
trouble at all with building for CentOS 7.3

I have what NVidia claims is the correct driver package, a 340 series. It
appears to build, but then fails to load. The only error I see is "no such
device", which makes no sense to me, esp. since it says nothing whatever
else.

I've gone through the install log, and there are a bunch of Note:, and
warnings, but the later I think are all about comparing signed and
unsigned integers.

And lsmod shows no nvidia drivers registered, but the logs claims that
Error: Driver 'nvidia' is already registered, aborting...

Anyone got any ideas?

       mark