i did the same conversion on my st141 although i used the dash from the cs-x. i dont have the car anymore and all my stuff is packed away as i just moved. from memory, there is a plug with a green connector on it that goes into the distributor. that is the wire you connect the tacho to. i just ran the wire along the top of the engine and into the same grommet as the speedo cable. if you have a gregory's manual there is a picture in one of the first few chapters that shows which wire to connect a tacho to.
the plugs are different so when you get the dash cut the plugs of the loom as well. the wires then need to be taken out of the old plugs (using a tiny screwdriver to lever them out) and put into the new plugs. the non-tacho dash has 2 plugs and the tacho dash has 3 plugs. you will also need an oil pressure sensor for the oil pressure gauge to work. i used the cs-x sensor so it screws straight in. i am not sure if the sensor from the 22r-e will fit.
i worked out what every wire did in both dashes from wiring diagrams and tracing the back of the dashes. you will need to splice an extra power wire and an extra earth wire (or two)as there are extra connections on the tacho dash.
the only problem i had was never being able to get the digital clock to work. i didn't really care though. a bit more effort would have seen it working.
if took about 1 hour to do that actual installation but many more working out where each wire would go. it was my first foray into auto electrics.
if you want the explaination of what all the wires do on both dashes i can dig it up for you. email me at trentomatic@hotmail.comas i don't read these forums much.
also with regard to diffs. the st141 cs-x also came with rear disc brakes. i was quoted $150 for a complete siff around 3 years ago.