So you have a non tach Mustang with gauges and...... you want to swap in this cluster with a tach.
Thanks to the efforts of Erich Bozzer and his article from the Boss 302 Registry here, the procedure is very straightforward and easy to accomplish. It is highly recommended that you read the article and it's step by step outline. The schematic shown below is meant to provide you with a layout of the pins and wiring before and after the procedure. The pinouts from a non tach car are in a different order, along with a few wiring differences, than the tach car connector. The pins from the connector are removed with a small paper clip inserted into each pin from the front of the connector and depressing the small clip or tang that holds the pin in it's location. These modifications will allow you to use a tach cluster with a non tach car.

There are some important results that you need to be aware of.
1. The oil pressure gauge becomes a warning light, therefore you need to change the larger bell shaped oil sending unit with the smaller sending unit used for a warning light.
2. Your three wire engine gauge feed wiring harness for the coil, temp and oil gauges may need to be modified. Remove that harness and check the resistance of the center wire, the oil sending unit. If it shows 0 ohms, then you can reconnect it and use it. If it shows 5 ohms, you need to replace the harness with one that reads 0 ohms or untape the harness, bypass the wire with a new wire and tape up the harness.

2. Your ammeter is no longer used and charge indication will have to accomplished by other means. There are important differences in the wiring harnesses between using a warning light or an ammeter. You can use a separate add on voltage gauge if you want to.

Before you perform this modification, you will need to disconnect the battery, remove the dashpad as outlined at
Dash Pad Removal, then remove the instrument cluster as outlined at Instrument Cluster. When you look at the connector, you will see the pin locations numbered.
 
The schematic below shows the wires in the connector with their color codes. The white wires are shown as light gray. The connector diagram on the left is the pin arrangement before the changes and the connector diagram on the right shows the pin arrangement after the changes have been made. The tach connection is made with the pink resistor wire cut where it exits the fuse panel before it enters the main harness. The two wire connector to connect the tach is a standard two wire trailer connector that can be purchased from most automotive supply stores. It will plug right into the tach wiring connector.
 
Factory Non tach cluster pin arrangement Modified cluster pin arrangement