As in Mopars the fifth Vin digit is the engine code. . I think 'S' may be a 390 car and 429's a 'Z' but am no Ford guy so confirm w/ someone who is? . Here's a link below to the free Mustang decode site....give it a whirl but I think you need the whole Vin to make it work? .
~ Pete
http://www.fomoco.com/mustang-vin-decoder.shtml