Fastest how? Top end or quarter mile? One of the fastest STOCK quarter mile Mustangs was the 68 1/2 (debuted 1 April 68) 428 Cobra Jet. Top end would have to go to the 04 Cobra with that cool guy overdrive. I don't qualify the Saleen as stock since it is BASED on a Mustang GT.
That's one Mustang fan's opinion on the stock Pony.