As far as I remember, there aren't any rules stating how large the wheel wells are allowed to be - it's more or less common sense not to make them INCREDIBLY large (it would just look wierd!)
64mm should be fine, after you cut them, place the body on the car and see how it fits. With the front wheels, turn the wheels and push the body to see if there is any possibility of the body being collected by the wheels, Same goes for the back end - push the sides in and see if it gets collected by the wheel as it rotates.
Car-less (Velox V10 or 733) // RB T10.5"+" (JB Modified) // Bluebird BMS-990DMG (St) & BMS-631 (T/B) // ATS 16% // Capricorn Wheels // Protoform
"70% of the time, it works ALL the time"