The tach would work fine, but it will take a bit of fabrication to make it fit in your Mustang's instrument housing as the Cougar housing isn't a straight drop into the Ford. The cup for the tach is shaped differently for the tach versus the two guages. You can slice the tach cup off flush with the rear surface and graft it onto the Mustang housing (after similar slicing off the guage cup). A wire-feed welder makes short work of the grafting. While tach and non-tach harnesses are differnt, adding a tach is simple. To wire the tach in you'll have to cut the wire leading from the ignition switch to the coil about 2-3 inches from the ignition switch. The ignition switch lead attches to a red with stripe wire on the tach and the black wire on the tach attaches to the wire leading to the coil. Factory tachs are wire in series, not parallel to the coil.
In the instrument cluster I modified, I also grafted an instrument cup onto where the clock sets and using a mix of 67 & 68 guages, had Fuel, Water Temp, and Oil Press across the top. I used the Cougar Speedo as well as the Tach so the slight lettering differences were not noticable between the large gauges. If you use a Cougar Speedo with trip, you can use the Cougar lenses, but their locating tabs are slightly off from the Mustangs, but easily fixed by filing a couple of small notches. Their shape is also different... not noticable in place. I used Mustang center buttons on the lenses inplace of the Cougar centers (attached with silicone).
I took just one evening to take care of all the surgery and wire mods.