In my experience, all you need to do is hook up the wires from the pcm to the gauges and everything will work (went from a '98 SSEi into a '86 LeSabre, and then into a '93 SSEi). Fairly easy to do if you understand basic DC circuits.
You will need the service manuals or access to the wiring diagrams. You will likely have to make a harness adapter for the firewall, where you can add any relays and whatnot that the '98 pcm wants. Things like the fuel pump relay, any security system wiring, OBD connector, cruise control module ignition wiring, power and grounds, etc where wiring will need to be added; and for the most part all the gauges, cooling fan relays, evap, A/C relays, park neutral switch, just need to be connected to the right wire. If you make a firewall adapter, you can limit anything that could be wrong with the wiring to that adapter.
On the older H-body cars the PCM was under the dash. I use the PCM as an adapter box for the harness under the dash, and made an adpater that plugs the donor harness into the factory firewall pass-through. That way if I ever decide to sell the car it's plug and play to put it back to stock. Like this:
You mostly need the wiring diagrams to check for differences between circuits (so you can hook them up right). An example would be if the check engine light on the '92 gets it's +12v from the PCM but on the '98 it get's grounded through the ECM. Another example would be the starter wire on the '92 coming out the drivers side pass-though, but on the '98 it goes through the passenger side pass through.