I think it was in mid 65 that change was made from 5 to 6 bolt, going by HP part numbers, C40Z-6010-C for 5 bolt-- C5OZ-6010-C for 6 bolt.
Timing bolt cover pattern on block is same , but design of cover in pump attach area is different to accept later pumps with sandwich plate ( early cover had direct contact with coolant in impeller cavity & was subject to erosion- also provision was made for oil fillertube to intersect with cavity from timing chain area to valley.)
So if your block cast number is C6--- then you have a 6 bolt 289.
Jac Mac