For anyone running factory EFI Engines like the LT1 & LS stuff, here is how to get all the gauges to work:
Fuel gauge and Volt gauge are in the car wiring and works.
Oil pressure: hook up the Sending Unit from the Engine to the wire for the gauge in the cluster.
Tach: wire PCM Red/Green Pin 10 in the LS Engine Harness to the Tach in the Cluster, set the pulse correctly in the tune. For the LT1 look it up in the link below.
Water Temp: need 1998 LS 3-wire Temp Sensor GM Part # 12551708, wire Pin C to the Temp gauge in the cluster. For the LT1 just wire the single green wire on the harness Temp Sensor connector to the gauge.
Speedometer: if using an electronic transmission, convert the speedo gauge in the cluster to electronic. Wire the VSS from Red/Green Pin 50 in the LS Engine Harness, can be calibrated correctly in the tune. For the LT1 look it up in the link below.
LT1 WIRING INFO:
http://www.lt1swap.com/all_lt1.htm