The easiest/cheapest front disc swap is the granada spindles. I bought mine for $40 at the junkyard (were comeplete, even had some of the upper/lower control arms on them!). Tie rods are the only thing that is different, plus they are stronger. Well the caliper is different too I believe, but as long as you buy all granada stuff pertaining to the front brakes you'll be fine. If you do a search in the forums you should be able to find some threads pretty easily that explain it a bit more. (check this out too:
http://www.geocities.com/2bav8/granada.html) For your rear perhaps you want to consider a versailles rearend, which I believe are only slightly wider than a stock mustang rear, but come with disc brakes and often times a limited slip rear. Like I said I have a regular 8" rear and have only read about the versailles rearend, so you'll need to do some more research on that one if you choose to persue it.