The owners' manual for my high school car showed a similar break in procedure. I followed that instruction after I rebuilt the 270 for my GMC circa 1980. Maybe it is a reason the engine still runs well 45 years later.
1950 GMC 100 Panel Truck, 1958 270, 3-spd Saginaw with BW OD, 3.73 12 Bolt Differential from a 1966 C10, Front Disc Brakes Hill Holder Bought it in pieces 9-1976, driving since 6-1978 Follow in the DITY | 
