To do it correctly, you will need to add some wires to the bulkhead connector in the engine compartment. Specifically, a tan wire to the negative side of the coil for the tachometer, and two wires, black and black/white, one to the junction block connector behind the battery and another to a splice in the harness on the alternator charging wire for the ammeter.
The temp sender wire can be used as is, but you will need to get a new temp sender and temp sender wire connector. I recommend using www.lectriclimited.com
part #06400759 for the sender, #VRK6871TS for the sender wire.
Looking at the bulkhead connector from the engine compartment, the tan wire to the coil negative is located in the inside connector block, at the bottom inside location in the block, next to the big purple wire. The wire goes from the connector into the engine harness that runs along the firewall and exits with the resistor wire over to the coil.
The black and black/white wires from the ammeter are in the other bulkhead connector in the next to bottom and bottom locations on the right side. The wires go into the harness along the inner fender. You will have to unwrap the harness tape from the alternator wiring until you get to a splice with 3 or 4 red wires spliced together. Solder the black wire from the ammeter into this splice and wrap the harness back. The black/white wire continues in the harness past the horn relay, under/across the header panel and attaches to the junction block on the radiator support in fron of the battery. I recommend adding a 22 ga fusible link to the end of this wire at the connector.
Double check what I've said about the locations/colors of the wires in the bulkhead connectors by looking at the interior side of the connectors for the tan, black, and black/white wires. Connect the wires at the firewall side so that they plug into the correct wires to the inside. You will need some type 56 terminals to crimp on to the end of the wires. They are available at NAPA in both male and female versions.
There may be other ways to do this job, but this is the way the wiring was done at Chevrolet, and I know it works, 'cause I did it on my car.