Under the heading of, 'Been there and done that'. I'd tell you to look for a '76 to '80 Lincoln Versallis. That is, if you want a bolt in 9", with disk brakes. It seems Lincoln used springs on those cars and they match up to the Mustang springs just fine. The pinion yoke, will probably need work, as may the shock mounts. That's really about it. The one thing you'll learn, is that they are 28 spline and usually high geared, like over 3:00. I in fact have one in the shed, it is a 2:50. Great for Bonneville, but not, for a get out quick, car. That's when you get an Auburn set up, or even a lower stock, open end.
As for the 8", the previous posts are pretty much right on. I am able to get well over 300 horses out of my Stang and with the stock 8" and the C-4, I have never broken anything. I do, however, get lots of wheel spin. That, is probably what saves me. The car barks the tires, hitting second, if I just push it a bit and let the trans, do it own thing.
Just look at the housings, on the two diffs, and you'll see, that the 8" is more of an oval shape. Once you see the difference, they are easy to spot.
Hope you can find something in all that, to use.
TJ