Several things I can think of off the top of my head is the 67 and older had engine had a different lifter and pushrod than the more common later style big block setup, the 68 and newer had the same size pushrod ends which i believe both were 5/16 where the 67 and older had two different size pushrod ends one being 1/4 and the other is 5/16. The older motors can be easily updated to the newer set up if needed.

Also 67 and older motors had closed combustion chamber heads where as the 68 and newer used an open combustion chamber head, I believe the piston compression heights are different.