I did a t-5Z swap in to my 70 XR-7. The link below is the story. Your project will be a little easier, being your already a manual transmission Cougar.
by DieselD »Basically you will need the transmission, bellhousing or adaptor plate to use your toploader housing, T5 shifter, clutch, Crossmember, > speedo cable or adaptor to convert to electric, > Ive seen the Z-bar reused but its advisable to convert to cable or hydraulic setup, slip yoke and driveshaft modification
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If you buy a New T-5Z from Modern Driveline or Summit Racing your old cable will work fine. Or if looking to get a used one just make sure it is from an earlier Mustang or Ford passenger car. T-5s were put in to Camaros and other brand cars. WHile they can be made to work, you have to modify them, save your self the heat burn. I had to replace my old speedometer cable as the machining tolerances were off ether on the T-5 or my old cable and it would not go in to the transmission. But I used a new reproduction one that was a factory type replacement. Funny how a few thousands can be such a head ache.
You may also want to get a different set of gears in the rear. Mine had a set of 3.0 from the factory. Overdrive was nearly useless under 70/75MPH due to too low of engine RPM. But 1st through 4th was OK. Much more fun now with a set of 3.5 gears.
Good luck with this project and I think you will love it.