Find a granada. Some had 351w's in them. Those are the best. You can get disc brakes, a 9" rear end and a 351 motor all at one place. One I found once even had the rear disc brakes on it.
Get the proportioning valve from the Granada and upgrade the master cylinder to a 2 resivore type. I used Mustang II brake lines to keep from re doing the hard lines up front.
Mercury Cougars are a great place to look. They are often overlooked and used the same exact parts as the mustangs; rear ends included. Cougars were more of a "luxury car" compared to the mustang back in the day, so you will find discs on these cars more often than mustangs (at least in my experience). I'm adapting '68 cougar discs to my '65 right now using a '74 maverick m.c. You can actually get new calipers for the '68 cougar/mustang very cheap from your local parts place which is a huge plus, but it is nice to get the rotors & spindles from the yard.
Granadas are a sure bet to find with the discs, but I have heard of wheel fitment problems with those spindles. Whatever you decide to do, good luck!
Thanks a lot for the help... I guess I'll take the boy to the junk yard and see what we can turn up. I am not really familiar with Granadas. Is there a certain year that we need to look out for? Thanks Again