Well, do not be discouraged. Sometimes there may be more than one build sheet in a car, and certainly buildsheets for other cars being found in car to which they do not belong is not out of the ordinary. Seat could have been swapped in from something else, perhaps? Did you check the lower part of the back seat, under the carpet, or behind the glove box? Where there is one, there could be a second... No build sheet, no problem...
Also, to echo 4spdragtop's observation, is the dash that is in the car a 150 MPH Rallye speedo or has it been swapped in?
