K the amp is not designed to run .67 in mono, it can do it BUT, and I mean BUT...if your voltage drops the older orions are known to fry. Honestly I wouldnt run the risk unless you are very confident that it will be ok.
Now as far as power, they were underrated are great amps. I own one of the 275s.
I would bet at .67 you are looking around 475-500rms, if the voltage stays constant. But that is a guess, no one can really tell you unless it is benched.