If you're looking for an elegant way to add a tachometer to your dash, you might want to consider the Classic Instruments optional Ultimate speedometer/tach combo gauge as well to your gauge setup. NPD stocks all three gauge styles with the optional Ultimate speedometer in the gauge kit too. Classic Instruments offers additional gauges in each line if you would like to add a clock or other gauge to an A-pillar mounting solution, or perhaps you'd rather have a separate tachometer for a Shelby gauge pod. Classic Instruments can handle that directly or through its dealers. Lastly, Classic Instruments offers LED bulb upgrades and LED indicators fitted to the face of the gauge. If you want a check engine light for an EFI conversion, or perhaps a small red LED in your temp gauge to let you know when your fan is engaged these custom upgrades are all easily possible through Classic Instruments' custom gauge program. Just give them a call for more details.
For those of you with an early Mustang that utilizes the Falcon style sweep cluster (or for you Falcon owners!), modifying your dash to install a five-dial instrument bezel isn't a big project. Compare these two photos of an early Mustang dash with its flat bottom, and a '66 Mustang dash with the clearance "notch" above the steering column opening. This area must be cut from your dash for gauge clearance and can be accomplished with a cut-off wheel, angle grinder, or even a plasma cutter (be sure to protect or remove all wiring, carpet, and so on if using anything that will throw sparks). Once you've made the notch, the only other modification is to bend the lower bezel mounting ears outward from their straight up position and you're good to go.
As far as wiring is concerned, the installation is the same, with the exception of having to cut the white/red stripe wire from the oil pressure warning light to use on the oil pressure gauge. You'll also need to separate the two turn signal indicator wires from your single turn signal bulb socket (so you have a true left and right indication). The only tricky part is the generator/alternator warning light wiring. Some people leave the bulb intact and leave it under the dash as a warning light (what we recommend), but if you really want to remove the light socket, you'll have to do some minor rewiring of the charging circuit. You can find several pictorial websites online that explain how to do just that if you want to go that route.