SM420 (reverse is to the left) does not require a rear mount. All support is through the bell housing. If you get the SM465 (reverse is to the right) which is a newer tranny, a rear mount is required. You can use your existing bell housing, but you will have to machine to enlarge the cutout. Bolts are in the same place, though. Either one is a good box, parts for the SM465 are easier to get, they were built from about 67 to 79. Iirc, they also take different pressure plates.
