Ok that gives us a better idea, the 15" rims will limit your number of kits due to rim size mostly, you won't fit any rotors bigger than ~11.5" rotors so all of the 13" rotor kits using late model mustang brakes are out.
You also need to pick a budget, how much are you willing to spend to upgrade the brakes?
We can start a list of what will be needed:
1. PB booster and MC - ~$300
2. PB pedal - $50-100 (you can reuse your pedal box by drilling 2 holes to mount and removing some backing nuts so the booster will mount, also have to enlarge the firewall hole for the booster.)
3. disc prop valve - $50-125 depending on where you find it, I would only buy new. The valve is different from drum to disc.
4. Brake lines -? may be able to reuse/bend some may have to be replaced.
5. Rotors, calipers, bracket, dust shields, brake hoses
Companies I have read about with kits that fit 15" rims:
1. Chockostang
Classic Mustang Disc Brake Conversions and Power Steering
2. CSRP
CSRP discbrakeswap Mustang Disc Brake Conversion Kits CATALOG
3. ?
There are companies that offer kits with the power brake booster and MC as well as various other parts so you really need to research what is in each kit before buying.
I would lean towards the kits that are like the original '67 - 4 piston Kelsey hayes but with the improved SS pistons, from what I've read the steel stock ones tend to corrode/stick so the SS are an upgrade.
There are Granada kits where you change out your spindles, outer tie rods, get a realignment, etc. There are also kits that use a mix of ford, chevy and cadillac? parts which is a hodgepodge to me, I don't know how I'd remember what came off what when it came time to fix something or change pads.
As you can see there are a lot of choices out there and others can chime in with more companies that make kits that fit under 15" rims.
At a minimum you will need to install the booster, pedal, lines, rotors, calipers and bleed the system.
Going with a stock type setup will minimize the number of things you need to change, they will bolt right up to your stock spindles.
I'll think of more later but as you can see research is important to get a kit you'll be happy with and can afford. You can search fleabay for kits too.
Jon