The pump assembly is comprised of several parts, the part that has to be compatible with the input shaft is the "reaction shaft support" (the part that unbolts from the main pump housing). The supports functionally* interchange from '62-'66 and '67-'77. '78 was the first year for the 3-tab thrust washer and the 3rd ring on the rear piston retainer; as you discovered, these won't fit earlier units.


* I say "functionally" because there was a change to the lube circuit in '74 but interchanging parts with earlier has little negative effect.