My suggestion is to not buy a 67mm Variable ND filter. Buy a 77mm ND filter and use step-up rings to fit the lens with the 67mm filter threads.

A 77mm filter will prove far more versatile. On that, I highly recommend the Singh-Ray Vari-NDs. I've had three different types of variable ND filters, and Singh-Ray is by far the best. It's costly, but worth every penny.