58-57 Edsels had them also. Bought at least 5 for $100-125. Also have bigger brakes on the back. About 1 inch wider than 66 stock and 1 in.shorter than 67-70. Know a guy that has 20 Edsels for $5000 for all but he will no longer part them out. I don't have room for 20 Edsels. Gary
Have your 8 inch housing converted to a 9 inch housing. This way you use your stock axles and brake set up. This is done by removing the 8 inch center and replacing it with a 9 inch center. This is what I have in my 65 fastback.
I agree with RTM, have your 8-inch axle tubes welded on to a nine-inch center. I had one built this way (3.25 Trak-Loc) with all the best go-fast stuff inside and a good center-section casting for about $1000 locally by the differential guy that all the Ford racers in my area use.
The Granada/57-59 solutions will certainly work, but I would offer the following caveats:
57-59 center sections aren't the best and are generally 2-spider setups - not really much better than your 8 inch, IMHO. I've also heard it is a mother to find brake stuff that will work with these rears.
The Granada housing is an inch wider and are hard as hen's teeth to find. A wider housing will limit your wheel and tire options. I looked for about 5 months, crawling under every junked H-code Granada in the state (at least it seemed that way) and found ZERO 9s. Did find plenty of 8.8s, though.
Both the 57-59 and Granada rears are likely to have highway gears, meaning you'll have to put in a new gearset (might as well do a full rebuild while you're at it).