I'm surprised you located that many that haven't been modified. The 55.1 is sort of an anomaly as it was only a partial year build and contained components from the previous year and changes like the open driveline on the next series. GM used the SM318 identifier on way too many transmissions in my opinion. 48-54 with a torque tube, 55-62 in trucks and 55-57 in cars with the long tailshaft, 58-64 in cars and 64-67 in trucks with the short tailshaft. It's no wonder it's confusing.
Fred
