many years ago when i convert a capri mk3 from auto to manual, i found that where the gear lever is on the tunnel, the manual has a round hole and the auto has a square hole, and i can remember something about the starter being on the opposite side to the manual, which also meant the battery was on the wrong side, dont know if the sierra is the same, and of course if you change it to manual you will need to change the diff as well, for a 2.0 manual diff