just changing the speedo gear is easy, on the driver side of the tranny towards the rear the speedo cable and vss sensor go into the tranny. one bolt, you pull the sensor/speedo gear out, pop off a clip, put new gear on, re install.
the problem witht he 90 up aod's and t-5's is the actual gears in the tailshaft of the tranny only accomodate up to a 3.55 speedo gear. what happens is if you use another gear it can or i should say will slowly chew the speedo gear up.
i did that with my old aod. i put 373's in, used the closest gear that would work for it, and when i did my t-5 swap and pulled the gear out it was chewed, still worked, but wouldn't have for much longer, and that was with only 1000 miles on it...
i'm not positive if thats the same with the 95 or not.......meaning, i don't know if there's a gear you can use thats just not as accurate, but will mesh ok, or, if it's the same deal, where if you use a 3.73 speedo gear it will work but eventually get chewed...
to totally fix it, you can buy the correct drive gear for inside the tranny itself and use the correct gear........how difficult that is to do i couldn't tell you......
i'm guessing it will be off by more than 1 or 2 mph, and that will get more innacurate the faster you go...