Your next step should be to post this question in the Windsor Power forum where it belongs.
However, since you're here, I'll reply.
Replacing the differential gears is the logical next step. I suggest either 3.73 (minimum) or 4.10 rear gears to compliment your automatic 5.0 Mustang. The actual gears (Ford Racing) are less than $200. Installation costs will vary but figure another $200. - and, for your own peace of mind, plan to have the install done by a pro.