Slant 6 head interchange: 1975 head on 1969 Block?

I wouldn't necessarily call a '69 block early. Any slant 6 head with the solid lifter setup will fit any earlier slant 6 block. The heads are the same for any block size. There were some changes in later years to the spark plug (which still works fine). The heads eventually went to hydralic lifters. These heads will also physically bolt on but won't work without some extensive oil passage mods (I think-never tried it). Also, around '67 (maybe) there was a slight change to the combustion chamber, but that works fine either way as well. Most believe that the later chamber design is better.