What I would advise you to do would just to buy a lower gear ratio and a limited slip. Something like 3.50 or 3.80. They usually range from $130-180. and about $400 for a good locker or limited slip. It would be much less work to do, and cheaper if you get it done by somebody else. than changing the whole rear and out. I'm pretty sure they didn't use the 8 inch rear end past the 70's. The third member that you have in your mustang is the stronger of the two designs so it should hold up nicely to whatever you do unless its going to be a race car.
If you want disc brakes what you could do is find a lincoln versailles rear end in a junk yard. It is a 9inch rear end usually with about the same ratio thats in your car now. They come with disc brakes and of course is a stronger rear end. The only problem with this is that its wider and would need to be shortened.
So i guess you have a couple of options here but, unless I'm mistaken i don't think any 8 inch rear ends came from the factory with disc brakes. you can buy good kits that start at about $500 bucks (stainless steel brakes).