ChuckDock,
The Kelsey-Hayes spindles you need are only available used. They are often found in this section:
Mopar Brakes, Steering, Suspension, Tires & Wheels For Sale
The brake setups available through Dr. Diff are different, and based on later style systems. Good stuff, to be sure, but different. If you're in a hurry, this may be the way to go.
Also, if you go with the Kelsey-Hayes system, you'll be limited to the 4" Small Bolt Pattern rotors as the only ones that are readily available. FABO members have had some success in machining Mustang rotors to use with the K-H brakes, and get the 4.5" bolt pattern:
Using mustang rotors for 4.5" bc
If you go the K-H route, you'll also need caliper to spindle bolts.