The main instrument pod (tachometer + speedometer) has 3 10mm nuts that hold it in. Two of them are sort of behind the steering column, facing the firewall. Feel around for them and you will find them. The third one is also facing the firewall, directly behind where the tachometer is. Once you get the nuts out you can pull it out just enough to remove the cable from the back of the instrument housing. when you have the cable removed, you can pull it out and have plenty of room to work with the wires. To take the gauges out, first remove the mounting bracket from the bottom of the housing. Keep in mind that the housings are fragile, and any any mount of pulling on it will break the screw tabs that hold it to the bracket. I have not found a pod yet that doesn’t have broken mounting tabs, but it doesn’t really affect the instrument pod that much.
To hook up the tachometer you will also have to put the unattached blue wire on to the negative post of the ignition coil. The blue wire is the tachometer signal wire. Look at the wires around your coil, it won’t be attached and will probably be wrapped up in the loom. You should find a blue wire with a “bullet” connector somewhere around there. On cars that had the tachometers stock, the other half of that connector has a short lead that connects to the coil. I believe the yellow wire is power.