You can run the car with spacers with confidence... but they MUST be hub centric spacers for the Crossfire... hub centric spacers fit on the hub snugly and allow the wheel to mount and be in balance. NON-hub centric spacers are a pain to get aligned properly to get proper balance.... that is likely why you have the vibration at 65mph as you were told.
If you search for hub-centric spacers you will find multiple sources/brands. Rob at Needswings.com sells them....he is a forum sponser.