The transmission is a 3 speed, unless it was replaced with a newer model. Someone would have told you that though.
There should be a data tag on the drivers door near the strike plate, which would have all the build codes on it.
http://bradbarnett.net/mustangs/
is a site that you can use for all sort of production data. That guy is a mustang data freak.
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1997 Mustang GT "The Freak" - 13.80 @ 101.70, 2.07 60'
1995 Honda VFR750 - not much @ really fast (actual data pending.)
1964.5 Mustang 289
Rice Haters Club Member #13