Since you are looking to change the brakes I imagine that total originality is not a concern. If that is correct and I was in your shoes the first thing I would look for would be a complete 68 Cougar/Mustang power disc brake setup. If I could not find one then I would look for a 69 or 70 setup. Just be sure to know exactly what year the donor car was as there are differences between the years for some parts. If you go that route be sure that you get the brake pedal, brake light switch, power booster, master cylinder, proportioning valve, front brake line holders that bolt to the side of the frame, spindles, dust shields, calipers and as much hardware as you can get. Some of the parts listed will only be good for cores, others will need rebuilding. I have some of those parts here. I believe that I have a fairly complete 69 and 70 setup here.
Another route that a lot of people used to use was the setup off a Mercury Monarch/Ford Granada. If you use the spindle setup from one of them as I recall you need to use 70 Cougar outer tie rod ends. Then you still need to come up with a power booster/master cylinder setup.
Randy Goodling
CCOA #95