redripper:
The Autometer cluster replaces the cluster in any '99 to '04 Mustang. They do not make a similar cluster-for-cluster swap piece for the pre-99's (I called them several years ago). You have to take the circuit board off the back of your stock instrument cluster and swap it onto the Autometer cluster. The board contains PATS info and other things that are linked to the PCM. One of the Mustang mags (5.0 or MM&FF) did a one page write-up a few years ago and the swap is pretty easy.
Now, if you're talking about replacing individual gauges, that's a whole different topic. You have to get the sensors installed (where required), locate the signal converters, route the wiring into the cockpit, install the gauges into the new faceplate (and other places, if necessary), and then test everything to make sure the new stuff is working properly.
HTH,
Chris