[Rory McNeil]

Mustang 8.8s are so common and cheap to buy, I wouldn`t waste a dime on a 7.5" rearend.
The stock 79 4 speed is a weak, very poorly geared 3 speed with an overdriven 4th, again, I wouldn`t waste any time on it.
Assuming this is to be a Stocker project, it is much better to buy decent parts from the begining. When I started racing my 85 Mustang, I was running stock T5 transmissions for the first year. By the end of that year, I had 2 broken T5s, and a box of damaged parts.