'88 - '93 would physically fit. The interior had a makeover for '94 - '96 before GM went to the Gen II w-body platform, so the dash is completely different (looks very, very similar to the Gen II). As for adding the tach, I THINK the 91+ already had the white tach wire in the harness, so it would be plug and play. If not, you'll have to tap into the white wire that's in with the bundle of wires under the ignition control module. That should be the wire to receive the tach signal. Here's a good discussion about this, and what else you're going to need:
http://www.w-body.com/showthread.php...t-Cluster-swap
