Bob hit it right on the nose concerning gear ratio and strength. In 65 I bought a 39 box with Zephyr gears from Ford Parts Obsolete which at that time was located in Bell, Ca. I put it in the 36 5 window I had at that time. The car had a fiarly strong sbc with a 4:11 rear from a 40 pickup. I had my share of kills on the street wuth it.