No one has stated it explicitly , so I will:
The V-8 which came out in 1955 had a different bellhousing pattern than the 6 cylinders of the day. In 1963, a new line of sixes appeared which used the V-8 pattern. So, the 216, 235 and 261 (as well as the GMC 228,248,256,270,302) shared the old pattern while the 'new' sixes (230,250,292) use the same pattern as the V-8.
