What are the differences in a 67 A-body 383 big block engine than in a 68 or 69

If your using an original big block A body K member and motor mounts then the block must have the two bosses sticking out of the front of the block on the driver side for the motor mount, not all blocks have them, some blocks have them but they aren't drilled. If your car isn't an original big block car and has the conversion motor mounts with a small block or slant 6 K frame, then any block will work.