Since you’re using box squats as a max strength exercise, there is no reason, at least, initially, to mimic a powerlifter stance. Remember, not all powerlifters lift with really wide stances.
Read some articles by Dave Tate. He has some good ones on the box squat. Initially, squat light and a little high. Once you find your groove, slowly increase weight while maintaining proper form. Then drop the weight and lower the box. In fairly short order, you will find your groove.