I'm sure this has been done before but I just did it today and thought I would share what I did with everyone.
I had the rear pads on my drum brakes shatter into little pieces then I heard about some Subaru XT's being in the local wrecking yard. I went and grabbed all the parts for the disk brakes on the XT (backing plate, caliper, pads, rotor) and swapped them into my 1979 Wagon originally with rear drums.
The backing plates needed to be ground down a bit to clear the larger stamped/welded axle carrier. I then cut a notch into the carrier so that I could access the caliper bolt since you can slide on the rotor unless the caliper is removed.
Be sure to get rotors for a 4wd if your converting a 4wd car :-)
Pics should be self explanatory: