pushrod, assuming that you have the original tranny in your truck, chevy reffered to them simply as a synchomesh transmission. The serial number is located on the side of the case at the rear edge of the trans cover. for your model year, it should have a "J" (year designation) followed by "N" if it was built in Saginaw, "O" if built in Muncie or "P" if it was built in Toledo. Then it would have numbers which would designate the sequence number of the trans.
