I have a 69 Mach 1 original tach in cluster and like to keep things original but I have made some modifications which I will list:
* converted existing tach with rocketman www.RCCInnovations.com. Luckily I was able to buy one cheap on ebay and converted that. what he does is replace the guts but face is the same.
* got the console pod and installed voltmeter and oil pressure it in the ashtray which is where the shelby's had them.
* for voltmeter i just tapped existing wiring that was nearby the console
* mustangs have the oil pressure sensor made for dummy lights. so i had to replace with oil pressure sensor made for gauge.
* clock stopped working but i found a place where you can buy remanufactured original guts if your willing to take that apart.
Let me know if you have questions? rob