Hi Dave,
If you have a 4" tub it should be approx 4 1/4" inside and 4 3/4" outside the back of the tub should be approx 2 1/4" at the transum, the sponson should be approx 2 1/2 " past the transum, I saw another post about this on another forum, the answer was to use only 1 side of the bracket...