67 down 904 had a small input shaft, and pilot on the engine end of the converter. This matched the size of the pocket in the end of the crank, for small blocks, and slant sixes. In 1968 the crank pocket, converter pilot, and input shaft got larger. This is why you cannot put a 67 or earlier engine, up to a later trans. You can put a later engine up to the early trans buy using an adapter ring in the end of the crank.
