This topic came up not too long ago - see attached link:http://www.camaros.org/forum/index.php?topic=281.0
I've done it and it was no problem. You don't change anything in the dash harness. The existing setup in a car with idiot lights has a temp line coming in already to the light, just need to take the light out of the circuit and run the temp line to the temp gage. (Also change out the termperature sending unit to one for gages). Ammeter and tach leads can be run as new wires from the tach and ammeter gage out through the firewall to the coil, horn relay and the charging circuit junction block near the battery.
If you want to set it up as GM built it, the tach and ammeter leads (3 wires total) need to be terminated at the proper locations in the bulkhead adapter under the fuse block in the interior of the car. (Temp lead is already properly terminated.) Then if you replace both wiring harnesses in the engine compartment (engine and front light harnesses) with harnesses that are for a gage setup, you'll pickup the tach and ammeter connections through the bulkhead connectors.
Fuel line connection is simply a matter of extending the existing lead for a fuel gage in the dash to get it down to the console.
The last item is to remove the existing oil pressure switch and connect the oil tubing from the engine through the firewall to the oil pressue gage. The existing lead for the oil pressure switch can be eliminated as it is no longer used. (if you change out the harness, one for gages won't have this connection.)
Check out the attached link and let's go from there.