Unless you have way of converting the signal from the cable, or use a sensor different from the gear drive/cable at the trans and can decipher that input, then changing the speedometer makes no sense.
There are very nice units, one of which was eluded to in my members thread, by another member.
As the guys stated, it's a hella lot of work and won't fit in with a single piece of the car.
Because rally car or race car, maybe.
There are usually many more point to the car that require attention first.
Mainly transmission/shifter, suspension (bushings, struts, upper mounts dry rotting, sway bars) and brakes.
If the car has a 4g6X platform, it already outperforms the stock rubber bushings and engine mounts, and the suspension will be lagging also.
My thoughts on where to put the effort to start.
I did install a massive 5 gauge pod cluster just in front of my main dash cluster, mounted directly on the top plastic of the column.
It works for me, and did not modify anything other than a $5 piece of plastic that is available on every car from 1987/89-91, anywhere in the world.