i have a 94 gt and i know that the tranny is probably different in your car than on mine but when i swiched over to 3.73's we changed the speedo gears in about 5 minutes. from what i remember my cousin just opened a compartment on the side of the tranny and swiched it out. i dont remember to clearly cause it was some time ago but it didnt take a long time.
That is the easy gear to do, inside the tranny, there is another gear on the output shaft that drives the gear you changed. To change that one you have to take off the tail shaft of the tranny, which requires a lot of other work.