Thanks for the advise. I am guessing that the most torque going through this coupler will be 40-50 ft/lbs with minimal torsional vibration. I had guessed that a 1/4" key would be able to handle this, but maybe not.
For those non-Ecomodder types, and for a bit of extra EV commentary, I have also started a conversion blog here, for those interested:
Project: Electric Booger