Go on over to thirdgen.org... Plenty of info on swapping in a 4th gen dash into a 3rd gen. In a nut shell... The dash will mostly bolt up. The shell that is... You will need to fiddle with the mounts here and there to get it to fit right. You will need to fabricate or hack up a stock upper dash cover. You will need to figure out the a-pillar trim. To use the 4th gen HVAC controls you need to do some custom work for the actuator cable. Then there is the wiring... You will need to splice different things in order for your gauges to work. Also I believe you cant use 98 -02 gauge cluster with your 3rd gen. You need the 93 - 97 cluster. Unless of course you already have an ls1 in your 3rd gen... then you can use the later cluster.
This probably didnt help you much... Really need to check out thirdgen.org for that info. A few guys over there have it completed and it looks/works great. Good luck!