I generally use SPEC clutches, but for stock or near stock cars I have never found a clutch that feels, drives and performs better than a King Cobra. Late Model Restoration supply sells an aftermarket King Cobra kit at a very reasonable price. That being said, I have had in my own car or had experience with almost every clutch maker there is for Mustangs, and nothing holds up like a SPEC clutch if you are making serious hp.
You wouldn't be the first person to come on here who claimed that they just bought a car with an aftermarket cam. If the car has stock heads on it, it's unlikely it has an aftermarket camshaft. The stock cam really is the best cam for the stock heads, and no one recommends changing it out. I think many people will claim that a car has a camshaft to explain shaking when it fact there is something wrong with the car. A vacuum leak will make a car act like it has a cam.
Kurt