If your local junk yard has an experienced counterman he can be very helpful in telling you what rear from what year Stang will fit your '79 model. He will also have access to the Hollander exchange manuals that they use to find this type of information also. He can also find out which models came with rear disk brakes.
I wouldn't necessarily buy a new 8.8 as there are hundreds of them in use so there should be plenty available used in salvage yards.